From that height, assuming near-level ground, the most difficult judgement will come on extremely close shots. In order to get a meaningful yardage difference, you have to be either very high in the tree or shooting downhill. I've had deer come in on the downhill side of a tree where they were 10-15' below the base of the tree. All of a sudden, actual distance is significantly greater then the distance your sight needs to be set at. The basic rule of thumb, is to shoot the horizontal distance to the target.