Mark I have found the on-line recoil calculators (like the one below) to be quite informative. One must add the weight of wad and filler wad to the "bullet weight" to properly calculate recoil. Wads are light but they are not inconsequential, the AA12SL weighs 34.1 grains.
My favorite 7/8 ounce load, 1200 fps, generates 11 ft lbs of recoil in my 8 lb DHE.
Up the shot charge to 1 ounce and recoil increases 27% to 14 ft lbs.
If I shoot a standard 2 3/4 dram trap load of 1 1/8 ounce at 1145 fps recoil increases 54% to 17 ft lbs.
http://www.huntamerica.com/recoil_calculator/