Gun fit is a very subjective thing. What is comfy for one guy may suck for another even though, externally, they look almost the same. The various positions are all slightly different in terms of fit and what is ideal for one may be a tiny bit off for the others. So much depends on your bone structure and muscularity and what you have taught yourself, or your parents taught you in terms of posture. Even your emotional state can effect this as body language will change with variations.
I have found that a gun that fits me well in the standing position works pretty well in all other positions but I spend years and years shooting trap and there, gun fit is everything as you don't aim a shotgun, you learn to mount it consistently and point the gun using your major muscle groups to swing the gun at a moving target. A different game altogether from rifle shooting.
Now, I'm in the category of skinny old fart. I'm 75 years old, 5'10" and weigh 160 lbs on a good day. I have nearly no muscles left that work well, get winded looking at a hill and have an assortment of injuries from years of riding, racing and occasionally crashing motorcycles. I no longer shoot prone as I can get down to the rifle but actually cannot get back up without a number of contortions and some pain. My shooting is confined to a bench with a bipod or bag to support the front of the rifle and another bag under the butt stock. My eyes aren't that good anymore either and I tend to get squinty if I spend too long behind a scope.
Enough prelude. Let's talk about fit.
Length of pull is the most basic element as everything else depends on it. Adjust this so your face naturally comes to the stock with adequate distance so your thumb is not against your nose and you can comfortably reach the trigger in you chosen position with the rifle held firmly in your shoulder pocket but not jammed in there. We'll come back to this as scope position depends on it.
Now, with your face against the stock, your dominant eye should be right over the center of the bore. The cheek piece should be adjusted to allow this without undue canting of the head. Your neck muscles need to be relaxed and you should not be turtling your neck out nor should you be jamming your face into the cheek piece to get your eye centered, adjust as needed. I'm skinny and have very little flesh over my cheek bones, others may have a facial structure more like a chipmunk and will need more offset. I also have a long neck and require more rise than most. This also requires a bit more drop in the stock so that the butt pad fits in my shoulder pocket.
Once this is set, you adjust your scope mounting so that the center of the eyebox corresponds to the center of your vision with the axis of the scope in line with your eye and you are not straining to look up into the scope (too low) or having your cheek floating off the stock to punch you in recoil (too high).
Some of the things mentioned above will require a buddy, or better yet, a stock specialist to evaluate your position and shape. They are out there. Check at a local trap club.
One last thing, that is grip shape and position. This will be effected by LOP and your arms and hand. Find one that allows you to get as natural a grip on the gun with your trigger finger unencumbered, I don't really curl my thumb around the grip and prefer to have it resting near the top of the grip, you may prefer that or may hate it. The idea is to avoid tension in the hand, experiment.
Good luck to you in your quest for an ideal fit. Sorry for the windy.
