20 feet is high enough if you've got some cover to break up your outline. You don't want to stick out like a sore thumb. I usually try to get 20 feet up, it not only will help with you conceal your movement, but it will help disperse your scent farther away from your stand location. In my opinion, your OK at 20 feet as long as you've got some limbs around you to break up your outline.