As an engineer I know from dynamics and physics that aim must be adjusted when angles are taken into account during a shot. However, this is only if you sight your bow in on a level surface. Just take the dang factor out of the equation and sight your bow in and practice from an elevated platform that is equidistant to that of how far up a tree you intend to scamper up. That way there is no thinking about "should I aim high or low or dead on". Of course I'm assuming that the ground you may hunt on is level as well with no angled shots above your position, but you get the point. If you really don't want to rack your brain over this, get yourself a Bushnell Scout1000 w/ARC technology that does all the calculations and fills in the formula for you and tells you what you should be aiming for. There are many variables and one does not have time to figure out the math in your head especially the pathagreum theorum and mass force calculations when there is a monster at 18 yds. Just aim an inch lower to negate the effect of gravity and I think you'll find you'll be alright for those close range shots.
"I enjoy and become completely immersed in the challenge and the increased opportunity to become for a time a part of nature. Deer hunting is a classical exercise in freedom. It�s a return to fundamentals that I distinctly feel are basic and right"-F.B.