This is actually very simple. The distance from the base of the tree to the deer is the only distance you need to worry about. Gravity will act on an arrow traveling parallel to the ground exactly the same over the same horizontal distance as it does travelling at a downward angle. The angle does not matter as far as the effect of gravity is concerned. Trust me on this. What does matter is how you release the arrow from the bow.

If you bend from the waist to account for the downward angle, your arms will be in the same position relative to the bow as if you were shooting level (ie.If you picture a line drawn through the middle of your bodyfrom your waist to the top of your head, your arrow will be perpendicular to that line, and your front arm will be too). If you don't bend from the waist, in order to accomodate the downward angle, you will have to drop your front arm downward to make a downward shot. This will change the position of your arms relative to the bow (ie. picturing that same line through your body, now the arrow will be at a downward angle relative to that line and your front arm will betoo). There will also be changes in the position of your back arm in the two differnt scenarios, changing your release. All of these changes in position will change your anchor point and effect your aim.

Having said all that, it would seem that if you sight your bow in to a yardage on the ground, and you have good form in the tree (ie. bending at the waist), the arrow should hit the same spot on your target at the same ground distance regardless of whether you are standing on the ground or are up in a tree (no matter how far up). While this is theoretically correct, I have found that in practice it is not exactly true, especially with broadheads. Therefore, I practice from a height equivalent to my treestand height with broadheads using the same arrows I will use for hunting.

Under these circumstances I have found that, for me, my broadheads hit a little lower than my field points from a stand than from the ground at distances 30 yards and over, and the same arrows with the same broadheads hit a little lower from a stand than from the ground at distances 30 yards and over. Point being that everyone releases their arrows a little differently under different circumstances, and if you don't practice shooting under those diffferent circumstances you don't really know what will happen when you do. After practice shooting all of my hunting arrows with my hunting broadheads under hunting conditions (including wearing the same clothes), I have a lot of confidence that when I am actually shooting at an animal I know exactly will happen at every yardage I will be shooting from, at every angle I will be shooting from. Physics matters a lot less to me than experience because the less I have to think, the better off I am when it comes time to squeeze the trigger.