When estimating distance, pick spots at your eye level. This will give a true shooting distance to the base of that spot. For instance, say you have estimated that a spot on a tree at your eye level is 25 yards from you, shoot your 25 yard pin if a deer is at the base of that tree. This helps to compensate for angle. Or if you have the money, go purchase a cheap range finder ( not one of the more expensive ones with angle compensation and some big time Pro hunter on the wrapper) and do the same. The range finder will just take away the guess work of your spots at eye level.