20 yards from the ground is the same as 20 yards (from the base of the tree) in a stand. the way it was explained to me (and what I believe) is that gavity works on the horizontal distance. therefore, no matter how high you are, the effect of gravity (drop of arrow) is the same. however, the tendencies forma stand (I don'tknow why) is to shoot high.

with that said, even if you are 15 ft. in the air, the difference between yardages will be negilgible. if you're up 50 feet, that would be a different story. (just an example)