In a nutshell; gravity only effects the arrow over the horizontal distance. That is to say that if, for example...you are hunting flat ground, 30' up in a tree, and a deer is 20 yards from the BASE of the tree...aim 20 yards, not the actual distance from you to the target (which is about 22.4 yards).

The greater the difference between you and the target (or animal), the more yardage you have to deduct from the distance between you and the target to make the shot.

Geometrically speaking, it's called the Pythagorean Theorum (sp?). "A squared" plus "B squared" equals "C squared". I'd suggest plugging "geometry" into an internet search engine for a site that'll show diagrams of how the formula works. It'll help visualize it better.

JRW