While 8in seems a bit extreme, yes, you will have to aim low whether your target is below or above you in elevation. We tend to measure distance from our eyeball to the target, which will be longer than the straight horizontal distance if we are above or below it. If you were 20 ft in a tree on level ground, a deer that appeared to be 20 yards from you would actually be at just under 19 yards, so the distance is negligible, and decreases the further out you go. Under 10 yards it can be significant, but aiming slightly low, an inch or two, should compensate plenty. As high as you are hitting from elevation though, I suspect a form issue. Just be sure to keep your anchor consistent and bend at the waist rather than the neck or back.