Actually, gravity works the same at an angle or on a level; that's why you've got to know your "true" distance from the target, not your "line of sight" distance. There are physics majors on here who can come behind me to give you the formula, but you basically need to be concerned with the distance from the base of your tree to your target -- not the distance from where you are 15-20 foot high in the tree to the target. Another way to do this (if you're using a rangefinder) is to range a spot from your stand at the equivalent height that you want a reading; it accomplishes the same effect.