RE: What tells you to start trailing immediately, or to pack it in and pick up the trail in the morning?
Don,the reason you don' t go high is because you are apparently using good form and bending at the waist.
I think Thundermug has his numbers posted slightly off.If a deer is 25 yards from the base of your tree(horizontal range),then shoot it for 25 yards.If the deer is 25 yards from where you are in the tree,across the hypotenuse(slant range) of an angle,then shoot it for 20-22 yards.Sight paralax will make a slight difference but not enough to worry with in a hunting situation.
And it is the pythagorean theorum.
I think all the others have covered your question quite well.When in doubt,BACK OUT.