So you need to find a head angle you can repeat and automatically lines the eye and anchor up rather than trying to line up an arrow with different eye to anchor angles. If you lean your upper body be sure to keep the head angle the same as the body angle changes. It's o.k. to lean the body at slightly different angles as long as the eye to anchor angle remains the same.
It is important to find a head angle that works for you and once found to always be conscious of that position. This is the reason why. You need to get the eye behind the arrow if you are going to line it up. Lets say for arguments sake the corner of your mouth is your anchor. Go to your mirror and look. Put your finger on the corner of your mouth and you will see your eye is pretty much directly above your finger. Leave your finger there and lean your head to the right. You will see your eye is no longer over your finger, but is to the right of the finger. Therefore if you had touched that anchor with your eye on the right of it you would move the tip right and miss to the right even though the arrow appears straight to the target.