The trajectory difference is often blamed for misses over a deer, but the reality is often different. In your case, with a deer 12 yards from the base of the tree and you being 30 feet high, the deer is 15.62 yards from you. The trajectory is 12 yards. I've shot a lot of slow bows in my life, but have yet to see one where the difference between 12 and 15.62 yards is a clean kill or a high miss.
Even at 20 feet high and a deer 20 yards from the base of the tree, the animal is 21.08 yards from you. At 15 feet up and a deer 15 yards from the tree, the animal's distance from you is 15.81. Long and short of it: don't worry about the trajectory difference so much as your form.