Any distance where there is 20 MOA of "down" adjustment. Simple answer - but that is the deciding factor, and that depends on the scopes range of adjustment and the final scope mounting to the rifle (IE the setup in total).

Assume (for this example) a theoretical setup where the scope has 50 MOA of vertical (elevation) adjustment. If we can assume that the mechanical center leaves 25 MOA in the "up" and 25 MOA in the down, then a 20 MOA base will leave the same setup with 45 MOA of "up" and 5 MOA of down.

This is a rather simplistic explanation - but this should answer your question....