RE: Shot a decent 8 - Didn't find it
IMO, you had a very steep angle to work with. You probably did take out a lung. Remember that a stand 20 feet up, means you were probably shooting from about 25 feet.
I'd use this as a learning experience, and you may want to consider lowering your stand. It's not difficult, but you have to have some decent cover for concealment.
Sorry to hear about the situation.