Post is kind of elderly, however, you need to figure out what LOP you need. As daft as it sounds, shouldering a yard stick like you would a rifle/shotgun and looking where your finger comes will tell you that. So will putting the yard stick in your elbow and looking where your finger comes.
It'd probably be best to buy a custom butt stock. A slip on recoil pad will add about an inch. You can buy spacers and grind 'em(best done with a bench mounted belt sander) here.
https://kickeezproducts.com/recoil-pads/spacers/
A sheet of plexiglass, while it'd be homely on a high end shotgun, will work too. Regular woodworking tools with blades for plastics.
"...too long..." Easier to fix 'too long' than it is excessively 'too short'.