I haved to agree with Isnipe and Deerbearhog. I usually concentrate on aiming center mass then dropping down a third more from there. This way I am basically aiming at the top of the heart. If I miss up to 4 inches low I am still going to take out the heart all the way to the base of the heart. If I miss high, Im still hitting both lungs, but not hitting too high where I will have to wait for the lungs to fill up before I get a good blood trail. Up until the last few years though I was mainly a gun hunter so aiming for that spot would give you a 4 inch room for air in each direction. As people said before though, "Aim small, miss small"