Paul,
I am a newbee at this Trad stuff, but gap shooting, as I understand it, is the style of shooting I use. This is probably because of my years of compound shooting and utilizing pin sights. For me, the analogy of hitting a 35 yard target by placing your 40 yard pin low on the brisket works for me.
I will attempt to explain gap shooting as best I understand it. First, you need to find your "point on" distance. This is the distance where when you place the tip of your arrow on the target, that is exactly where the arrow will strike ( Point of aim = point of impact ). For the sake of this example, lets suppose that distance is 40 yards. Your arrow will arch 20 inches at 20 yards, the greatest distance of differential. So, at 20 yards, you would have to hold a gap of 20 inches low. Now, at 10 yards and 30 yards, your arrow flight is the same- 10 inches low. Therefore, at 10 yards and 30 yards, your holding point is the same, 10 inches low. This is because the arrow is still rising.
The distance that you determine to be your "point on" distance, whatever that distance is, if you divide that distance by 2- This will give you your maximum distance in inches to hold low. Example: "Point on" at 30 yards, at 15 yards you would hold 15 inches low. Your 10 yard and 30 yard hold would be the same at 7.5 inches low.
I hope I haven't totally confused you. One last thought. If I understand it correctly, you can confirm your maximum arrow gap by taking your known "point on" distance, in this example- 40 yards. If you stand at 20 yards and place your arrow tip on the center of the target, your arrow would strike 20 inches high. If you measure that distance and they should match.
Hope this helps,
Dennis