Head Position
Objective: To prove you have a good head position and do not require any stock work to be done.
Skill level: This is an advanced exercise and should only be performed under supervision.
Time: a detail and a committee member.
Background
 If you have to strain your head to position it in the middle of the rear sight then you will not shoot as well as you could.

If your head does not naturally lie in the middle of the rear sight then likewise you will have to put in more effort to ensure everything is A OK before each shot.

It can be difficult to determine if your head is relaxed and in the correct place.

This exercise is intended to help you work out if your stock is correctly guiding your head into the correct place.
Technique
This exercise MUST be supervised and you MUST stop if you are not able to retain the shot on the correct aiming mark.  The committe member will choose the mark you have hit.  If you cant hit it then understand your head position is really poor and other exercises  are needed.
  • Remove the rear iris from your sight.  If you are experienced at this you may choose to remove your rear sight completely.  If you are unable to hit the correct aiming mark you may just want to open the rear apperture as large as you can.
  • Set yourself up for a shot on the selected aiming mark.  Due to the large size of the rear apperture (or none) the foresight picture will be fuzzy.  Survive with it.
  • Once you have the OK from your supervisor - execute the shot.
  • Where did you hit?

The place that the shot lands will give you some idea of your "natural" head position. 

If your shot falls to the left then your head is probably too far too the right, vice versa for shots to the left.

If your shot falls low then your natural position is too high - the stock needs trimmed.  If your shot falls high then your head position is too low.

If you have an adjustable stock you may be able to make some alterations. 
If you dont have the luxury of an adjustable stock then you need to revert to other techniques.  If your stock is low then pad it up with cardboard and sticky tape until you find the correct position.  If your stock is high you may need to get permission and a sander! Left and right is harder to adjust for.  I cant help you
If you did not hit the chosen aiming mark or within (say) 1/2" of it then STOP.  DO NOT PROCEED.