Gravity will affect the arrow the same at any angle it doesn't matter. Trust me I just got a 100 percent on my physics test yesterday and it involved this. The angle of the arrow would affect air resistence but I doubt that would cause anything.

You are actually shooting the distance of the angle not the base of the tree to the deer, so unless the very minimal air resistence is causing you to shoot hight then this is not the problem either.

The main thing is bending at the waist this should solve most of the problem.