The accepted way to check the fit of a shotgun is to fire at a steel plate which is given a coat of whitewash between shots and a careful note then made of where the pellet charge strikes in relation to the mark you’re firing at.
The trouble is that if there’s any suggestion of ‘aiming’ or sub-consciously ‘aligning’ the gun muzzles with the mark as the shot is taken then the results will be misleading.
Worthless, in fact.
A gun that fits properly prints its charge where you are looking so you need to stand 14 yards in front of the plate, look at the mark with both eyes open and then bring the un-mounted gun to your shoulder.
The trigger is squeezed as soon as the stock meets your shoulder and face.
By standing reasonably close to the plate you are better able to determine the mark left by the shot because it will still be tightly bunched and not starting to spread.
Fire 5 or 6 shots (whitewashing the plate after each one) and measure any gap between the mark and where the shot ends up.
Hopefully you will find the charge covers the mark each time, in which case there’s little point in having the stock altered, unless of course you want the gun to shoot a little higher.
If you are happy with the way you mounted the gun and fired but the charge hasn’t hit the mark then some adjustment is probably going to be needed.
Hand the measurements you took from the plate to a gunsmith, tell him how far from the plate you were standing, and he will be able to determine the amount of cast or drop that’s needed.