Just starting to play around with mid range shooting. There's a bunch I don't know but simply don't have time to read the vast books of knowledge that's out there. I may talent more seriously in the future but as of now I have to ask one simple question I believe in over thinking. If a scope is FFP and substensions are in MOA,  then a 1moa line on the reticle would cover approx 1in at 100 yards. That being said if my vertical subtension is labeled 3 for example, and a particular load drops 1in per hundred yards, and I'm shooting at 300 exactly, then use the 3 moa substension and I should be dead on right? All conditions assumed perfect.

Yes 1 moa substention will be 1 moa no matter what with an FFP scope.
As to your drop I don't know. If the drop is 3 moa at 300 then yes 3 moa substentions would be correct.

1" per 100 yards is a half assed guesstimate that I have no time for.

Just to understand whats happening.
The moa/mil amount is multiplied by the distance. It is an angular measurement so as the distance grows the amount 1 moa is grows with it.

1 moa @100= 1.047"
1 moa @300= 3.141"
3 moa @300= 9.423"
3 moa @1322= 41.524"

As stated below, none of this math has to be done. It is quick and simple to let your scope or ballistic calc tell you how many moa to hold or adjust for.

If you are going by what a box of ammo tells you then I guess you have to do the math, it won't be very accurate.

I guess I don't totally get what you're asking.  Bullet drop is non linear, and thinking in inches at all is a huge mistake,  it doesn't matter if you're in Mils, or MOA, you want to use the reticle as a ruler and think in the angular units.   There is absolutely no reason or benefit to thinking in inches.  I see new guys do this all the time.  If you need 1 MOA you need 1 MOA not 3".

Why are you not thinking solely in terms of whatever your scope is set to run(MOA _MIL)...We were shooting to 2000 yards this weekend, and using the reticle to make corrections by MOA(scope is MOA)..had nothing to do with inches....All I ever use for corrections are the hashmarks on the reticle if I can see bullet impact...

This is what I was trying to determine. I was getting confused. Say a given bullet is zeroed at 100 yards and drops 1 moa every 100 after. So at 300 you would be 3 moa low. Where in was getting confused on is if your give scope has a 3 moa low substension, do you hold on that to get poa/poi at 300, or option b...
Since 1 moa is 1 in at 100, and your shooting 300, if the bullet drops 3moa at 300, if you had to mulitply the 3 moa substension 3 times which would actually be 9 moa (technically -9 moa).

First of all bullets don't drop 1" or  1moa every 100 yards after 100. They drop at increasing amounts every 100 yards because they are slowing down.  From 100yds to 200yds it may drop 3 inches but from 600yds to 700yds it could drop 40 inches. This because the bullet, traveling slower now, takes almost twice as long to travel the 100yd distance between 600 to 700, gravity is pulling the bullet down for twice as long making the drop a greater amount.

Now MOA, MOA is just an angle, like 5 degrees but smaller. 1 MOA is 1/60th of one degree so 60 MOA would be a 1 degree angle. Minute Of Angle.

Now if you take a line at an angle and extend it out in front you starting at your chest, it will be about 5 feet off the ground at your body, the farther away you get the higher off the ground the line will be. The angle has not changed but the length and height has as the angle continues on. So 1 moa at 100yds covers 1" and the same 1 moa covers 3" at 300yds.

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If you are just working with something like this and want to adjust or hold for it. Then you can do the math.
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Looking at the 300 yard spot, it says the drop is 8.5" with a 200yd zero(+2.0" @ 100 ).

So we know 1moa is 1.047" @100yds
At 300 yards you take 1.047 x 3 = 3.141"

So now we know at 300yds 1moa will be 3.141".

Now divide the 8.5 inches of drop by 3.141 to get the moa you have to hold or dial.

8.5 / 3.141 = 2.7moa on your scope.

Great info, I'm learning. So is this scenario either dial in 2.7moa of come up or use the 3moa subtension holdover and I should put it where I want her to go correct?

Yes, just shy of the 3moa substension.
This is pretty crude though, going off the information on the box. I wouldn't rely on it for hunting, it would get you in the ballpark.

If you are actually getting into longer range shooting then spend $100 on a chronograph, get the velocity of your bullet out of YOUR rifle and put that info ito a ballistic calculator to get your hold overs/scope adjustments.

This will get you much closer, the chronograph will pay for itself in a short time because you won't be wasting ammo "walking it in" and missing.

That is a really good way to justify to price of a Chrono.

If dialing is an option I would much rather dial 2.75 than hold 3 on the reticle.

I've been seriously looking at getting a chrono. Anybody have one they could suggest? I've been looking at the magneto speed, I like the magnetic attach feature.
Is dialing a scope in any more accurate than a holdover provided you use exactly the same amount (i.e. exactly 3, not 3ish).

Magnetospeed is a great buy.  Very accurate and easy to use.  I like that and the labradar but the labradar is even more.

I look at things from the perspective of a prs shooter so I may have a different view on things, but I always dial my elevation when I can.   Sometimes there times where you need to shave time and I will hold.   If you scope tracks right holding and dialing are the same theoretically but you're often guessing a little between hash marks

In the original post the bullet somehow is maintaining a 1MOA trajectory like a laser beam so you would use the 1MOA holdover or come up at any and every distance.

As already pointed out MOA is not exactly 1 inch per hundred yards. The inch per hundred is known as Shooters MOA and is for making quick or large adjustments or for shorter range shooters where the decimal point doesn't make much difference.

Chrono you're gun/ammo combination and a shooting app will do the math for scope adjustments and holdovers.

As far as dialing or holding over, the human eye/brain has a natural centering ability and most reticles have more detail in the center crosshair. The optical clarity is best in the center of the scope as well. So while technically they are both equally accurate, it is easier and instinctive to use the center crosshair. Dialing takes the guesswork out of where to hold, no need to count subtensions or guess where .7MOA is.