Probably over thinking it. But hey might as well ask in case there is something I wasn't aware of.

The irons have 150 and 250 marked on the sights.

So is there a certain procedure to zero this guy?

Or just throw a group down at 50 and call it good?

With the wide variety of front sights. I would get it dead on at 50 and check it at 150/250

I have a 16" para as well.  I set my aperture to the 250 setting and zeroed at 50.  It was on at 200 with a lollipop hold and a little low at 300 with a center hold.  I suspect both distances would be just fine with a center hold on a man-size target.

300 is about all I want anyhow out of irons

Perfect. Thanks for the info.

I'd zero it at the 250m setting, with preliminary zeroes at shorter distances at that setting to get in the ballpark.  The 16" barrel will ensure that one setting or the other will be off somewhat if the other is on.  With it on at 250m on the correct aperture, holding over, you should be good to go out to around 300m or so.  Id then shoot it on the 150m setting at 150m and see how far high or low it is at that range and just keep that in mind.  Maybe see where shots land at other ranges of interest.

The fixed rear sight used on some paras (and the Dutch-contract fixed-stock rifles) was meant to be zeroed at 250m.

I have my 21" Para zeroed at 150m, so the sight settings are both dead-on.

I believe that's the barrel length these sights were meant to work with.  I suspect the same for the para tangent sights (which I've only seen sold as parts but not on any example of a rifle made by FN so far).  The fixed one used on paras is of course usable for whatever barrel length one desires, since all elevation adjustment is with the front sight.

I did find a load that out of my 17" barrel corresponds fairly closely to the trajectory of NATO-spec ball ammo out of a 21" barrel.  Pretty much right on at the 2, 3, and 400m settings.  At 5 and 600m, it's at most 3 inches high.  Hornady 155-grain ELD Match.  If the other Hornady cartridges using this bullet have the same average velocity, they should also do the same thing, just with a bit less precision.

Im just going to mention that 150 and 250 meters are somewhat odd ranges to zero on

Is there a particular reason why they manufactured the sights to be in those ranges?

250m is a good range for a battlesight zero.  With 7.62X51, you should be able to aim directly at the vital zone of a target, or hold over or under a bit, and hit it up to that range and somewhat beyond, and most engagements with man or beast will be within 300m or so.

Not sure about the 150m setting.  Probably more convenient for zeroing, though, if you're using a barrel/load combination that has a trajectory corresponding to that for which the sights were designed (NATO ball/21" barrel).  Sight in at 150; while verification is always preferred, under those circumstances it should be good, or at least close enough, at the 250m setting/range.  Keep it on 250 for actual use.

Sounds like a 50 yard zero on the 250 setting and then check dope on steel out to 300 is the answer.

Ill play around with it a bit

@chase45

I would zero for 250m.

At 21-22 yards, POI should be almost the same as POA compared to 250m (making it a good range to get a preliminary zero/get in the ballpark).  At 50 yards, POI should be about 2" above POA for a 250m zero.  A 250m-zeroed rifle at 300 yards should hit a bit more than 2.5" below POA.  Holdover should remain reasonable out to perhaps 330 yards or a bit more (which is a bit over 300m).  Drop will be over 9" at 350 yards.  Drop rate increases rapidly past that range.  Highest POI relative to POA would be at 145-160 yards or so; about 5.2".

A 50-yard zero would hit 14.5" low at 300 yards.  At 200 it would more reasonable, being close to 3.5".  POI will be the same as POA at 127-128 yards.  Less than 1/2" high at 100.

Data is based on DAG-manufactured NATO ball (M80-style bullet) out of a 16.25" barrel.

You'll get the most out of your rifle, I think, by actually zeroing at 250m, or something close to that, and then seeing where POI is relative to POA at 150m on that setting (just so you know), although the 250m setting will be useful for pretty much the whole of your usable range.

300-yard zero should still have reasonable hold-overs out to 350-360 yards or so, if you wanted to stick to yards.  About 6 1/4" low at 350.  Highest point would be a POI about 6.6" above POA in the 155-175 yard range or so.  I wouldn't try zeroing at a longer range than that with that sight.

Also, take note that the sight adjustments for zeroing are based on meters.  The windage screws move the sight over 1cm at 100m per click.  I believe the 16-click-per-rotation front sights are the same in this regard.  12-click ones exist as well, IIRC, which I suppose would do 12.5mm at 100m.  Front sight posts also come in five different heights.  If you can't get zeroed with a particular post, or you are at or near one extreme or the other, switch to the next highest or lowest sight as appropriate.  Repeat as necessary.

Thanks Ill certainly do that

Appreciate the knowledge bomb. It certainly will help me get the most out of the irons

Hopefully this weekend I can get out and zero it properly

@chase45

Also note the bit extra I added about the front sights.  It may or may not apply to you once you get around to zeroing.

I'd try to get in the ballpark at 21 yards or so, looking to have POA and POI match.  Try again farther out to see if the POI is where it should be relative to POA at that range (100 yards, maybe).  If all is well, then put it out at 250m and verify that POI equals POA.  If it does not, adjust until it does.  All using the 250m setting.  Then place target at 150m and see where it hits using the 150m setting.  Maybe experiment and see if you can find the range where POA=POI on that setting.

Data I used is based on an M80 style bullet with an average muzzle velocity of 2624 fps out of your barrel length.  The ammo you use to zero may vary a bit, so it'd be good to check out a ballistic table based on the appropriate data and adjust accordingly so you know where to do your short-range zero (should be close to the above, though, if using some form of regular ball ammo), exactly where POI should be relative to POA at longer ranges, actual hold-over or hold-under, etc.

I've been using the trajectory calculator below:

http://www.shooterscalculator.com/

And the following estimator for velocity out of shorter barrel lengths:

http://shootersnotes.com/Vel308Win.php

Find the data for what you're using, estimate the velocity reduction, and input it into the calculator.  If you do the 250m zero, have the chart displayed in both yards and meters, and input 273.4 into the yards box.  I like that the calculator allows you to input multiple loads, with the trajectories compared on the graph, and a chart for each produced.

OP:
I prefer simple zero's vs complicated ones...….

Here's what I'd do, decide what your longest distance you will shoot, then look at one of the numerous ballistic charts and find the trajectory/zero that fits you best.  
Then pick one of the apertures and zero.  Done and Done.

For example, here's what I did with my SAR 48.....  I decided that the MAX I'd ever shoot this was 300yds.  Looking at the charts, I found I liked a 300yd zero.  
Why?  Because in flight between 0-300yds.... the highest it goes is about 5 3/4" high and then drops to zero at 300.  
So all I do is aim-center-mass thru out, knowing that at worst I might hit a little high.  Done and done.
BTW at 50 yds.... you need to be about 2 1/4" high..... that'll give you a 300yd zero.
Done.

There is no need to convert metric to yds , or whatever.  The sight adjustments work regardless, so if you work/think/shoot yards just use those.  
The rifles were originally designed for Europeans shooting on European ranges, who use metric.  
But you don't have to use metric to use the rifle.

I established a rough zero at 20ish yards today

I'm kinda scratching my head however. My rear sight is cranked pretty far to the right.

Every way I've tested the levelness between the front and the rear seems pretty dead on. Honestly I feel I have a pretty good eye on what's straight and what's canted.

It looks nearly perfectly straight. I've stuck a rod through the handguard retaining screw hole. Looks straight. Flipped the gun upside down on a flat surface. Straight.

But the rear is certainly cranked over to the right indicating that the front sight should be canted.

Furthermore the degree I'd have to crank the barrel down to center up the rear would certainly make the front sight canted off to the left

Well I either need an eye test or something else is amiss.

I suppose as long as it zeros its not a big deal. Just kind of a eye sore

Might be the barrel itself that has a problem.

I'm wondering that a bit.

I'm also wondering if the lower is tweaked

I dont think I mentioned it in this thread but the receiver and lower took a minor kaboom awhile back. I replaced the damaged parts and trucked on. Cant say for certain shat happened but I learned my lesson about letting guys use their reloads in my gun. Long story short its almost as if it slam fired. This was back when it was in a 11" pistol configuration

Who knows though. I dont see anything obviously off

It's certainly possible that something was damaged.

I know my buddy has to have his rear sight all of the way over to one side and it's still not quite zeroed, and the elevation is still a bit high despite using the tallest front sight post.  Barrel has a concentricity issue, according to Mark Graham at ARS (surplus South African barrel cut down to 18").  I don't know if DSA's barrels have a reputation for problems like that.

good  way i have found to time a fal is with a laser bore sighter.  crank the barrel and rec together till you think its good.  then put the laser in the bore. attach the lower with the rear sight in the middle and snapped together .   point the laser at a wall about 30 feet away or more if its aligned the laser dot on the wall  will be on top of the front sight post kinda like a red dot co witness.

if its off tighten or loosen barrel as needed, may take a few tries to get it perfect. you only need very little to be off for the need to move the rear sight a bit.

"if it shoot to the left it needs more heft. if its shoots to the right its too tight"   old saying from way back in the heyday of fal building . seems you are may need to tighten the barrel a little more

Yep one would think

I have a laser bore sight. When I first timed the barrel I noticed the boresight was left of the sight. But the degree Id have to rotate the barrel would likely leave the sight canted.

Im not sure what Im going to do just yet.

I suppose Ill just enjoy the gun for a bit. Rear maybe cranked to the right, but itll still shoot fine

With the sights on different "sections"...… is there much play between the upper and lower?  
What I mean is with the rear on the "lower" and the front on the "upper"..... could you shim it to push it one way/ the other?

Like shim the locking piece that comes down into the lower to lock it????  
See what that does for it?

I'm going to have to pull mine out and play with it...…………….

Nope it locks up tight. It was pretty loose before and I added some material via a tig to the latching component

I went the extra mile and even stoned it to ensure everything was flat and locked up correctly. The only movement I get now is the tiny tiny bit of play where the front of the lower attached to the receiver. Its so minimal Im not sure shimming it would do much.

Im just going to enjoy it for now. I can worry about the rest later once its new wears off

Yes, if it's shooting well and you can get a zero, I wouldn't fix something that isn't broke.
But if it were me, and I did start #$%^ with it to more center the rear sight, I'd look at how much/little and where you added via TIG to the latch.  Since you are pretty confident the barrel is timed correctly, I'd look at the easier (and known issue with the FAL's)
question as to if the sights were somehow off set from one another.....

PS:  If the rifle is shooting good groups, could you not test the "level" of the sights (and therefore what ever cant might be present regarding the barrel timing) by carefully tracking the rear sight, shoot groups and see if they are level.
IE.  Crank the sight all the way one way, shoot a group.... center sight, shoot a group..... crank the sight all the way the other way, shoot a group.  
If the groups are good and you are a consistent shooters..... the groups should be level (in a horizontal line)…..
That to me, would be easier than pulling the barrel on/off in an effort to check the timing  of it.
Besides It's shooting...……..

if you have the tools i would just retime  it.  you will be surprised at how little tightening you will need to do to move that dot over the sight.   i have tried all the methods throughout the years and the laser to me was the best