All, I've started loading for an HK91 clone. It has a Zeiss Diavari-DA 1.5-6x scope with nobs cammed for M80 ball ammo out to 600m in 100m increments.

I'm working up a load with Hornady 150gr FMJBT bullets. I don't care about matching the load's trajectory with the BDC, but I do want to know what the zero distance would be for my load at a given nob setting. For example, I'd like to know if I turn the nob to "2", then my load will be on at 239yds or something.

What is the best way to do this? My idea was to set up a target at 100yds, fire a round, turn nob to the next setting, hold the same point of aim, fire, turn nob, etc. Record the vertical offset, and then use this to determine what the nob would equate to down range. Then confirm at the actual distance.

Am I on the right path? Anyone ever done something like this? Thanks!

If you have access to a chronograph you could streamline the process quite a bit by punching some numbers into a ballistics app like Strelock or Shooter. I've found both to be accurate enough to get me on paper out to 600 if the data entered is solid.

So many variables are in play no factory BDC can ever be perfect. If memory serves me right H&K 91's have 19" barrels. That alone is oddball enough to induce errors in trajectory estimates.

I would choose a more accurate bullet if you actually intend to shoot longer ranges.

Sierra's, Hornady's and Nosler 155's will shoot smaller groups and have a ballistic coefficient and BDC very close to M80 when you take the your shorter barrel into consideration.

Develop the most accurate load you can using a match grade bullet design. Once you're established that, provided it feeds and functions 100%, you can address the scope settings.

I would suggest zeroing the rifle @ 300 meters using the scopes 300 meter setting. This will balance out the scopes BDC settings specifically to your rifle and ammo combination.

It may be a little off up close or further out, but you'll be very close across the entire spectrum of the scope's BDC markings.

Well said, especially in bold

The easiest (and best) way to get zeros is to shoot the rifle out.  Take what ever load you have and shoot at 100yds, get a zero, shoot 200yds and get a zero,  etc etc.

The next easiest way is to chronograph your load, zero the rifle as far out as you can and then use the book answer to GUESS the other zeros.

Anything after that is a poor attempt at a guess.  

And yes............  since the BDC will be off no matter what........... shoot a better bullet.

There will be several problems with your proposed technique.  

1.  Aiming error and group size will confuse the data.  

You will need a lot of shots to know a realistic group center.

2.  100 yards is not far enough to separate the groups.  

Given that each setting is only a few MOA apart, your groups will overlap and blur into one big mess.  Try shooting at 200 or 300 yards/meters, instead.  This will provide a larger separation between groups.  At 200 yards, the 100 and 200 yard settings may still be too close but give it a whirl.  

Recommendation

I'd say shoot groups at 100, 200, 300, 600 and possibly 400 yards, if your range has that.  Keep your zeroes for each distance and compare to values on knob.

I agree with TGH456E.  If you are somewhat serious and are shooting longer distances, get better bullets or you are "wasting" precious range time and your money.   You'll still learn, so have fun, no matter the outcome.