On a link from Dr Andrews' site yo can get a fair number of pages from his book on line.

Synopsis is that shotgun patterns seem to follow the law of Gaussian distribution which means that roughly 60 per cent of the shot is in the central 1/3 of the max spread of the pattern. This is observed in all chokes, spreads and distances. Ergo you try to put the center of your pattern on the target and forget the fancy stuff, in other words technique is paramount, ballistics not so much.

This was a very rough synopsis.