I would highly recommend that you buy 'Masters Of The Barebow' video.
I never learned my gaps to the point of knowing exactly how many inches below or above the target I needed to be.
I mainly use my arrow to make sure my left and right are dead on and than feel my gap by being consciously aware of what the total sight picture needs to look like.
Rod Jenkins, who is one of the masters in the video has a system to determine his exact gap by inches for different distances.
Rod discusses finding your point on distance, which you stated was 40yrds. and than shoot at 20yrds. with the point of your arrow on the target and measure exactly how high you shot. The gaps are than determined for the specific yardage between 0 and 40yrds.
What some archers don't realize is that learning to aim using the gap method helps an archer become more accurate quicker especially at longer distances and can eventually become very instinctive like to the point an archer may learn to use those gaps to become totally instinctive.
I personally think it is the best all around aiming method an archer can learn for both competition and hunting.
Ray