You are correct that the distance is actually longer when elevated. Hypotenus (spelling) of the triangle. However, since you are shooting down gravity has less effect on the arrow so you should shoot it for whatever the horizontal distance is (physics, the effect of gravity on a mass). There is always the chance that you'll shoot high. This has nothing to do with the distance but with shooting form. When you practice and sight in your bow your are standing in a "T" form. So to keep your shooting form correct when shotting from an elevated position you should bend at the waist to keep the upper body part (arms and shoulders) in alignment. Hope this makes sense to you.