All my gaps were established using NFAA field archery targets. That's the only kind of bow shooting I've ever done where I actually used a gap system. See if I can remember... It's been some years now since I shot NFAA.

I adjusted my arrow length and anchor so that my point-on was at 50 yards. 45 yards was the point on the bottom edge of the target. 55 yards was point on the top edge of the target. 60 yards was the shelf splitting the bull. 65 was the shelf even with the top edge of the target. 70 was shelf on the top edge of the target, read the tip of the arrow, mark where it hit the target butt (or tree limb/weed on the hill/cloud/ whatever behind the target) raise the shelf to that point.

40 was point on the bottom edge of the target, read the shelf and mark where it points on the target (or on the ground in front of the target), drop the point to where the shelf marked the gap. Double that gap read for 35. I didn't have gaps for 30 yards and less; shot those instinctive. 80 was by guess and by gosh.

I had a REALLY complicated gap system for my compound that used the arrow tip and shelf, as well as the top, middle and bottom of a springie rest. I had aiming references for every distance from 5 yards to 80 yards. All memorized because you couldn't use written notes in the bowhunter class.