To figure out the usable range take the vertical travel of the scope and divide by 2; add 5.8 MILS for the additional travel afforded by the 20 MOA base, and it will give you the amount of usable travel for a given scope/base combo.
So for a Vortex 5-20 HD:
Total vertical adjustment: 36 MRADs / 2 = 18 MRADs
Add the Scope base, in that example it's 20 MOA: 5.8 MRADs
Total vertical is: 23.8 MRADs
Then use your ballistic calculator (here is a free one: JBM - Calculations - Trajectory
) to determine how much adjustment is needed to reach your desired max range.
The down side is the image quality/fov degrades as the internal adjustment approaches the max (in either direction).
My personal feelings are it is more important to have the internal adjustment bottomed out at the zero, and have the scope closer to the center of the adjustment at range, and hence the clearer image at the distance; Rather than the other way around. I feel it is more important to have the highest quality image the further out, than close in.