Head position is one of the last things that archers master. The head should be as square to the target as possible. Once it is set in place or after addressing the target it should remain as motionless as possible. Easier said then done though. Personally, I have to do neck stretches a few times a week in order to get into the correct position without straining. The best sight picture will come when the pupil in your right eye (right handed shooter) is as close to the center of your eye while at full draw as possible.