There was a big discussion on another site not long ago about whether or not folks need to adjust their yardages from a treestand. In the end, I swear some hunters must take a slide rule in the woods with them.
Think of it this way: Suppose you're 21 feet up and the deer is 20 yards out. The deer is actually about 21 yards from you, but the trajectory of the shot is only 20 yards. Now, I've shot some awefully slow bows in my day, and none of them were so slow that the difference between 20 and 21 yards was anything more than an inch difference in impact, at most.
If you're shooting considerably low out of a treestand, odds are it's a form issue, not a trajectory problem.