The Army simulates 100 yards by zeroing at 25 -- each click then moves the impact 1/4 inch. Start at 11 clicks up and zero windage, the average for the M1. SHOOT THREE SHOTS. See where the center of the shot group is, then move the sight to bring it to the point of aim in stages. When you have it, leave the windage alone but count the clicks as you move elevation down to the bottom. That will give you your zero. Put a piece of tape on the inside of the butt trap door, write your zero on it as Up 13, R 3 or whatever number of clicks it turns out to be. You can then always return to zero by running the sight down to the bottom and adding the right number of clicks of elevation.