A lot depends on the downward angle, distance and the deers state of nervousness or total at easeness.
I have aimed as low as 2 inches below the belly line on a deer at 18 yards and achieved dead center hits. I' ve also aimed right where I want to hit. It' s a learning process. We all have to miss once or twice to realize what' s going on. I usually aim somewhere in the lower third of an animal broadside.