Rather then guess on your arrow drop, this chart shows you the drop from 20 yards to 40 yards. The average arrow drops a lot more then a couple inches from 20 to 30 yards. You can expect 6-8" drop from even the fastest of bows.

Speed (fps) Drop in inches
220 28.7
240 24.2
260 20.6
280 17.7
300 15.5
320 13.6