i was always told inside 20 yards to aim low, about an inch from the bottom. i thought this was stupid so i tested it out shooting from my second story window, i aimed at the center bull and put 3 into basically the top side of my block.

EDIT: but im sure how low you aim is determined by the angle, 85% of our sets are 20 feet high just to give you a bit of a visual. hope this helps