ORIGINAL: Badatta2d
It's the horizontal distance that matters. You can neglect height. So if you draw a right angle triangle and are shooting along the Hypotenuse, ignore the verticle leg and calculate distance on the base.
I shoot ranges with my laser range finder to trees on my height level.
True, I should have mentioned that I use a laser range finder also, so I know the exact distance.
The distance from your stand is longer than the distance from the base of your tree. Keep that in mind.