I was always told, & it seems to hold true, that the level or horizontal distance is what matters. 20 yards looks farther when 20' up a tree but I'v paced off 20 yards & climbed into a tree & hit my mark. It seems the closer the target gets to the tree I have to start aiming lower & the farther away the less it matters. I think the hardest part is estimating distance from in a tree. Good advice is given above, practice from a tree, pace off distances around your stand and or buy a range finder.