here's an old skeet shooters two bits
i am what most people call huge, so the average gun doesn't fit me and i have alter a lot of stocks, the process is the same for nearly everyone.
step one: is the gun too long or short? if the thumb of your trigger hand is hitting you in the nose, the gun is too short, no matter what the distance from the crook of your elbow to the trigger. if it is more than about an inch between your thumb and nose, it's probally too long.
step two: once you have the length established, what do you see? the back of the receiver: stocks low, build it up with moleskin, trick pads, a new stock, etc. daylight between the beads, seeing a lot rib or indications of too much height....go see a good stockfitter, let him help on this one.
step three: are you a thick chested person? you might try adding washers under your top screw in your recoil pad so the heel does not dig into your chest. this also affects how the gun recoils into your face.
i have a beretta 687 eell skeet gun that i absolutely could not stand to shoot in the 12 or 20 until i added about 3/8" of downpitch by adding washers and made it quit hitting me in the face as the gun recoiled off the point of the heel of the gun. when you move the place the gun recoils closer to the center of the pad and higher on your shoulder,in other words, make the gun kick straighter back and less up into your face, the gun will be much more pleasant to shoot.
cast-off can also help keep a gun from digging into your chest and get your head aligned behind the barrels.
try the gun after each modification, see if your are heading the right direction. there are more things which you can do, some simple , some not, but these are good basic starting points.
sorry i got so long