If the deer is 30 yards from the base of the tree, then hold point on.
If it's 30 yards away on your line of sight angle then you're looking along the hypotenuse of a right triangle. You know two sides, the height and the length of the hypotenuse. Solve for the third side. In your illustration, it's 29 yards.
So, unless you want to be solving trigonometry equations while sitting in the treestand [:'(], make measurements from the base of your tree to landmarks around your stand. That way you'll know that bush is 20 yards, that sapling is 25. That rock is 35. You won't have to be guessin' and cypherin'.