I spent an interesting afternoon with a Korean War sniper. He said that his target was always just below the adam's apple. Assuming no windage he said that 6 inches either way and you had a kill. I'm sure others have better info/experience and I welcome their comments.
Ah, because the shot can go high or low based on distance, and it's hard to judge exactly, but it won't go left or right without wind. Makes sense.