Forget about the slugs as all of them will kick fairly hard.

If you have a 3" or 3.5" shotgun I would suggest starting out with something like 7/8ths ounce skeet loads then a standard field load, heavy field load, turkey load, 3" turkey load and then a 3.5" turkey load. This will give you a very wide spectrum of the recoil pulses from lightest to the heaviest.

Then you could calculate the actual free recoil energy with the various loads and place the recoil number with the corrosponding imprints of the butt of the gun. I would also place the foam, clay or what ever against a hard immovable surface like a wall. It will give much better imprints.