How it should fit
The collar should fit comfortably on your neck when you stand up straight with your arms at rest. There should not be a bump behind the collar, or a gap between the back of the neck and back of the collar.
How the neck posture adjustment works
Changing this setting moves the collar forward or backward on the shirt relative to the shoulders.
Determining the optimal neck posture setting
To check this you will need a friend (or several mirrors). Often-times, if you try to self-diagnose this one, the very act of turning your head to look in the mirror will create or hide the very problem you’re looking for. With the collar buttoned all the way up, stand up straight with your arms at your side. Look straight ahead and be sure to keep your shoulders relaxed and comfortable (don’t push your shoulders back or hunch them forward at all).
In this position, if the back of the collar is pushed tight against the back of your neck and the fabric of the yoke of the shirt is pushing up on the back of the collar you will want to consider either the “Neck Back 1/2” or “Neck Back 1” adjustments based on the severity of the bump.
Alternatively, if the back of the collar does not even want to touch the back of your neck, you will want to try the “Neck Forward 1/2” or “Neck Forward 1” adjustments.