First is there any particular reason you have to stick with a 80 grain bullet? I like, and won't use less than, 100 grain bullets on deer with a 243. A heavier bullet would retain more downrange energy than the lighter one as well if range is an issue. With a 100 grain bullet I have killed deer out to 300 yards. I agree that a good rule of thumb is the 1000 footpounds of energy level. A 100 grain 243 bullet is still above 1000 at 300 yards. The 80 grain has less than 1000 at that distance.