RE: Shooting from a tree stand...
VA and chef are right. Pretend there is a triangle from your stand to the target to the base of your tree. Gravity only affects the horizontal portion of the arrow flight. So the correct yardage is the distance from the base of the tree to the target. Then bend at the waist to make the shot. If you absolutly can't practice beforehand, I'd take an extra arrow with a field point and take a shot into the base of a nearby tree when you first get out there. Make sure you spray the arrow down with no-scent before, and just leave it there to avoid stinking up the place. At least then your very first shot won't be at an animal.