In an ideal situation for me i always want to be between 20 and 22' up with good cover, I hunt an area where deer come in with their heads up lookin in the trees all the time and i just feel a little more secure. That being said, that is ideal, and as we all know the ideal situation is rarely the case. I think you can deffinately get away with 15 with cover around you, lower than that i just start feeling like i might as well be sittin on the ground due to all the mid canopy underbrush.