Here is my method for a 3D course...let's say for a 33 yd shot.
1) Is it over or under 30 yd based on my first impression.
2) Where is the halfway point. What is that distance? Over 15yd less than 17yd. So full distance should be 30 to 34 yds.
3) Take ten yard increments...1, 2, 3, plus about 2 to 4 yds...so we are about 32 to 34 yds.
4) Shoot the shot for 34. High ten. Check with rangefinder, 33 yd.

This method is pretty effective in most situations on level ground. But from a treestand, I find things look further to me than the horizontal distance, which is what dictates which pin to use. Am I hurting myself by doing 90% of my field judging practice from the ground?

Two things come quickly to mind... well, maybe three.

One, you can never hurt yourself by improving your yardage-estimation skills.

Two, take those skills and just estimate the yardage to the object you want to shoot -- but at the height you're at. There's usually a tree (in my case anyway) close enough so that you can look at it on a level playing field, if you will.

And three... when I'm in a treestand, I usually range a few key landmarks (twisted trees, etc.) from my stand before the hunt really begins so that I'm ready when the deer does walk by. That's been my most effective utilization and most oft-used method for my rangefinder. Of coursse, if you don't have one... keep working on your yardage-estimation skills!