I don't believe that Howard Hill ever memorized gaps and distances. I believe he started down the path toward split vision by wanting a method for improving on his first shot. He no doubt had almost perfect form, so he could be confident that wherever his first arrow went, his second arrow would also go if he held the same for his second shot. The problem was, what if his first shot went low or high? He found that if he noticed where the arrow point was when he fired his first shot, then if his first shot went low or high, he had a point of reference to adjust for his second shot. This is a short hop away from always noticing where the arrow point is for any shot, and placing the arrow point in the same relative position for any shot where the distance seems to be the same.
What is really the difference between “seems to be the same” and “seems to be 30 yards?” The difference is that “seems to be 30 yards” requires cognitive thinking, which takes your mind away from awareness of the bow, which is only possible when thinking non-cognitively. This is fine if you want to hunt with a range finder or shoot at marked distances. But if you want to hunt without a rangefinder or shoot at unknown distances, I believe split vision works better than gap. I think this is really what Howard Hill meant when he said, “You have to decide what you want to do, boys, hunt or shoot targets.” (Probably not the exact quote). If you want to aim using split vision, I see no benefit in taking a detour through gap.