I am cornfused here
I thought you just looked thru the hole and put the front sight on the target and let fly?
Your eye automatically Centers the Front Sight in the Hole and all you need to concern yourself with is the placement of the Front Sight on the Target .
The Target and the Rear Sight will be out of focus but the Front Sight should be in focus.
Hard Focus on the Front Sight!
I can see how gross changes in your cheek weld could change the focus of the Front Sight, when looking thru the aperture, but small changes aren't going to make much difference in focus, and will make no change in where the bullet will go. They will only affect how long it takes to acquire the target.
Once sighted in correctly the gun will shoot to the top of the Front Sight,,, Period..
I can assure you that if you look thru the Rear Sight and put the Front Sight on the target, the bullet will go there as long as the gun is sighted in in the first place.
The procedure for sighting the gun in has no more variables than adjusting the front sight until the bullet hits at the top of the sight. Head position doesn't change this because you have to look thru the hole and your eye automatically centers the Front Sight in the Hole..
The whole purpose of the development of the Aperture Rear Sight was to reduce the number of things you have to focus on when sighting the gun from 3 with Open Sights to 2 with Peep Sights. Then the Optical Sight reduced that to one.
However in practice the Front Sight is all you need to concentrate on. I know it sounds counter intuitive as virtually all of us learned to shoot with open sights by shifting focus between all three points. As you get older you won't be able to do this, but even in the Military now they are teaching to focus on the Front Sight and forget the rest.
Jeff Cooper was the main proponent of this and his teachings were responsible for others to build shooting schools around this very concept. One is actually called
"Front Sight." Goto www.frontsight.com
for more info.