I was wondering about this myself, I did practice from my stand but I was using my range finder, I just shot at the yardage my finder said(range from treestand to target).But I here people say range a tree or something from stand at the same height does it matter if you range at an angle? does it need to be at the same height? it didnt seem to matter as I was hitting fine..Its a cheepo finder, no arc or fancy stuff. Also I agree about the stand practicing, its a different ball game in a stand versus just shooting @a target. I just started bow hunting and it is tough enough learning from the ground let alone in a tree stand
For the distances you are talking about and the height of the stand, the difference between horizontal and what the range finder says will be under a yard, so don't worry about that.