15-20 my first deer though i was only about 7 feet off the ground because the tree i selected had a split in it and i was to lazy to change trees
One thing I've come to realize is the tree controls a lot of the height feeling as well....you put me 30 feet up a oak that has branches everywhere it is gonna feel shorter and "safer" than just 15 feet up a no branches Poplar. A buddy of mine was scared stiff to go higher than 10...last year he picked 2 oaks with tons of branches and went 15....he felt just fine with all the branches encircling him.
I tend to hunt around 14-16', but that is based more on comfort and the height my sticks safely take me to the platform. I pick a tree with care and wear an ASAT leafy suit, which helps a great deal with concealment. Several folks I know are leaving the trees and going back to ground hunting using leafy suits and pop-up blinds, so it is really personal preference.
The shot angle only becomes an issue when they are in tight to your tree. If they're out 20+ yards I don't think it's much of an issue.
My average height is 12-15' only because I don't like being any higher! Every year I tell myself "another foot" and I just can't do it!
I tend to place my stands up high like 25' for a few reasons...main one being that is the way my woods are set up...mature older trees form the canopy then you have the understory and it takes getting up into the 20+ foot range for good visibility without spending all my time cutting down shooting lanes and brush. The other reason I like to get up that high is to take advantage of wind and thermals better. I feel like in certain situations I can get in tighter to trails and push less than ideal winds as my scent drifts out over the top of areas vs settling down in them. However if I find the right tree with the right back cover I would hang the stand at any height. even 5'. I will say that I think you can get away with a lot more motion at 25' than 12-15'. Deer seem to scan the woods visually and can catch that movement in their periphery up to about 15-20'. Above that you have to be dancing around to get seen.
15-20 but would actually depend on terrain/cover. I have a 20' pull rope I use so that when I am climbing, when the bow starts to lift off the ground I know that I am plenty high.