The answer to your question is in the scope specs. The reticle is usually centered in the FOV of the scope.
How much of the grid you see at a given magnification is defined by the FOV. The FOV is usually listed in degrees or feet at 100 yards. The reticle grid is usually in mrad (unless you are one of those perverts who uses MOA reticles
).
To convert from degrees to milliradians, multiply the number of degrees by 17.45.
If your scope's FOV is described in terms of ft at 100 yards, you have to do some basic trigonometry: FOV(mrad)=1000*atan(ft*3/100). That is an approximation, but it is close enough.
Given that the specs are listed only for top and bottom magnification, you can calculate an intermediary magnification starting at the top mag. For example, for the 7-35x ATACR, the FOV at 25x is approximately equal to the FOV at 35x multiplied by the ratio of magnifications (i.e. 35/25).
If you are comfortable with excel or google sheets, it is very easy to make a spreadsheet to do this for you.
Now, keep in ind that on the lowest magnifications, you usually can not see the smallest subtensions. Also, since most manufacturers cheat a little with FOV at low mags, the calculations above typically work well for the upper three quarters of the mag range. I am also assuming that the magnification settings are properly calibrated.
Oh, and Frank, as far as the anvils go and all, welcome to my world...
As a part of my dayjob, I have to explain how electro-optical systems work to all sorts of procurement agents from the government and prime contractors. About a third of them are perfectly reasonable people. And then there are the other ones.
ILya