i consider it to be where when i'm at 30 and 40 yards and shoot at a dot near the top of my target with my 20 yard pin, the arrows fall directly below the dot vertically. first i have to be dead on at 20 before doing it. if my arrows are going to the right of the dot from 30 with my 20 pin i move my rest left a little. if left, vice versa. this determines my centershot, and if you can shoot consistently from 30 and 40 it will work for you.