Howdy,
Out of necessity, I have done some experimenting using filler wads in the shotcup. I first began downloading the 20-gauge for my daughter when she was eleven. I used Alliant Unique data and .125" 28-ga wads to load some really effective 3/4 ounce loads. Losing 1/8 ounce of shot made a BIG difference in recoil and it cured her of a developing flinch from AA's in a light SxS.
From messing around with the 12-ga, I discovered that 1/8" of 20-ga filler wad equals 1/8 ounce of shot volume. If you are using a standard 1-1/8 ounce shotcup, 1/4" of 20-ga filler will bring you down to 7/8 ounce. I loaded a box of 5/8 ounce 12's for a man who was recovering from surgery, couldn't allow recoil to damage himself, but still had garden pests. The worked great!
Check out the online auction sites for fiber wads. I have a good supply of Alcan and Winchester waxed fiber cushion wads that I picked up for pennies. Another place is Circle-Fly wads. They cut fiber wads in every gauge for use in muzzleloaders, brass shells, paper hulls, and for shotcup fillers. Another good source is
www.ballisticproducts.com. They have everything under the sun and dispense great advice, too.
I strongly encourage you to investigate the filler wad concept. They're cheap, and if you can find a good patterning 7/8 ounce (or lighter) load, just think of how much farther that bag of shot will go!
Good luck,
Bitterroot