We may be talking semantics here, but it sounds to me like you're all using "Point of Aim" rather than "gap". Or, you're looking at the "gap" that you see between the 'Bullseye' and some spot on the target butt or ground in front of the target.
I shoot split finger and anchor with my index finger at the corner of my mouth. My gap is about 3" @ 20yds; 2" @ 30yds; 1.5" @ 40yds. My point on is 65yds. In other words, I focus totally on my intended impact point, and out of my peripheral vision I see my arrow and visualize the gap in inches between the "spot" and my arrow.
Of course, the starting gap for any shooter is the distance from the nock to the shooter's eye, measured in inches. For me, that is just over 3"; and that's the gap I'd use For distances less than about 18yds, because, for all practical purposes, most decent bows will shoot 'flat' out to about 18+/-yes. Of course, distances closer than about 5 yards are a whole different story; but that's subject matter for another thread.