Gordon,gravity has the same effect on the ground as it does 20 ft up in a tree.The reason most people hit high from a tree is because they drop their bow arm.This changes their draw length and sighting plane.Trust me,if you're in a tree,aim strait in front of you and then bend at the waist keeping your upper body in the same position.If you do this,you'll be dead on.Bow speed has no effect on this.If your target is 20 yards from the base of your tree and you bend at the waist,your 20 yard pin will be dead nuts on.