Well, redclay, I just started gap shooting about a month ago but I'll give you one tip that helped me. I don't think of the gap in inches or feet. I use the size of the target as a reference. Let's say your gap needs to be 15" at 15 yards. Let's also assume that you are shooting at a deer which we'll say is about 15" from back to belly. If you want to hit the deer in the heart, just put the point "one deer width" below the heart and shoot.
This method seems to work on different size targets too. For rabbits, you might have to shoot "two rabbit widths" low. I'm still experimenting with this method but it seems to be working well so far. After a few days of shooting the gap becomes almost automatic.
Good Luck Joey