I won a rifle in a giveaway here on AR5.com.

I want to zero it in at 300 yards but have no 300 yard ranges close by.  I do have plenty of indoor 25 yard ranges close by.

I boresighted it at 300 yards with my mildot scope.  Scope is 4-16x40 and the mildots are accurate when magnification is set at 10x.

The crux, the rifle has a 20MOA rail on it and I'm hoping I can get the scope on target with a 20 MOA rail on it.  I don't intend to loosen or remove the rail.  Will I be able to get on paper at 25 yards with a 20 MOA rail?

A couple of ballistics calculators show I should be 0.7" low at 25 yards for a 300 yard zero.  Do the ballistics look correct?

You can get close but it will be a matter of luck to nut a longer-distance zero at 25yd.  Between parallax error, horizontal offset mounting, and simply the resolution of impacts at 25yd, you're likely to be several inches off at 300yd, but in the ballpark.  You can zero at 25 and be 'good enough' to set up paper and tune it in at 300yd.  Putting a very EXACT optic height over bore into your calculator will help.

Yes, I did measure accurately the distance above the bore of the scope objective and put that into the ballistics calcs which allow that entry.

The calcs I used were all within a 1/10 of an inch.

It helped knowing the exact ballistic coefficient of the bullet David Tubb's used and the fps his loader tries to achieve with their factory/custom ammo.

I have pretty accurate numbers but it the factor is the 20 MOA rail already being on the rifle an me not wanting to remove it.

I can't find the exact total adjustment of the scope I have so that's a big unknown.

Unknown maybe but not for long...  Turn the elevation turret as far as it will go in one direction and write down that "setting" (in this example, say it's 4.6 mils).  Then turn the elevation turret as far as it will go in the other direction remembering how many times you pass the 4.6 mil mark and how much farther past the the 4.6 mil mark is the "end" of your ability to rotate the turret...  do the math and you will have the total (vertical) adjustment.  

Looks like I have a total of 344 clicks of elevation adjustment in my scope.

Each click is 1/4 MOA at 100 yards according to the manual, even though the scope reticle is Mildot, this should give a total of 86 MOA of adjustment assuming each click actually does what it is supposed to do through the entire range.

What I've seen before on other various scopes of various brands is at the extremes of the knob/dial rotation (i.e. completely up or completely down in the rotational adjustment range of the knob) sometimes don't move the reticle a full 1/4 MOA per click since the knob gets real tight to move at the extreme up and down positions.

I'm going to assume the complete range of physical adjustment works properly even at the extremes of the knob/dial rotation and that's where I come up with 86 MOA for 344 clicks of total adjustment.

I'm going to center the reticle in the adjustment range and see where it points compared to bore sighting through the barrel just to get an idea where the physical center of the reticle adjustment is at when looking at a target 300 yards away.

Then I'm going to bore sight through the barrel at 25 yards and see how much different that is compared to 300 yards.

From my calculations I have approximately 25 MILs of total elevation adjustment with approximately 12.5 MILs of UP or DOWN adjustment from the center point of the reticle since I manually centered the reticle in it's range of vertical motion. (i.e. 172 clicks from the full UP or full DOWN to the center point of the scope's elevation adjustment.)  That's 12.5 MILs from centered reticle to the full UP or full DOWN adjustment.

So the 20 MOA (5.82 MILs) rail gives me 5.82 MILs less downward adjustment of my scope reticle compared to a 0 MOA rail.  This means I really have 6.69 MILs of downward adjustment of the reticle compared to a 0 MOA rail since the 20 MOA rail is pointing the whole scope down by 5.82 MILs.

What I mean by downward adjustment of the reticle is I can only move the reticle physically vertically UP in the scope tube the equivalent of 6.69 MILs which would move POI down 6.69 MILs.

I hope what I've calculated is correct.