OneOunce gave you the best description he had .... the problem is, there isn't an "exact" way to do this. Some of it comes down to how the gun feels in your hands. I think if you cut the stock to about 2" from your thumb / and try it out ...before cutting another 1/2" you'll get it figured out. Then go another 1/2" if you need to. If it isn't quite right / you can always compensate with a different thickness of recoil pad too ( either slimmer or thicker ).
The other thing I might suggest - talk to some guys at your local range / see if you can mount their guns / assuming someone has a shorter length of pull than your gun - and see how it feels. If that isn't practical / go to a used gun shop / where they have a lot of guns ...take a tape measure, measure the length of pulls ...and see what feels right.
Most of us, as we mount a shotgun, have a "natural" move forward, then up and back, as we mount the gun ....and I have the same move in the field that I have in Skeet, Sporting clays, etc ....or at least it works for me. Going to a different style recoil pad / a little slicker or something - might help keep the gun from hanging up / or just practicing your mount at home 25 times a nite will help as well.
I went thru a lot of shotguns before I really got my length of pull dialed in / to the point where I know exactly how I want a gun set up ....and my length of pull doesn't change for a 10 lb 32" barreled Trap Gun O/U .....to a 28" O/U Field gun at 7 1/2 lbs ....but there are things that make it feel better ( the type of recoil pad, is there a palm swell on the gun, the style of the forened...)..