You don't mention what you are using for powder. If I was pursuing this I would look for the highest pressure low speed load in the book and interpolate for the desired result. I would start with a 15 percent reduction in powder and go to the chronograph for fine tuning.
The problem is that you NEED pressure to get proper burning. When pressure is too low a load will get filthy with unburned powder.Maybe if you call Winchester they will help you out.
You can find the load you want on Hodgdon's website. Win. AA hull. Win AA wad (CB-clone wad ok) with a 1 1/8 oz. of shot. Followed by 12.3 grains of Win.AA Lite powder. For a load that is 980 fps. Good Luck to Ya. break em all Jeff
My light load for 20 gauge is Win AAHS hull, CB 3/4 ounce 20 gauge wad, 11.2 grains of Green Dot, Win 209 primer and 11/16 ounce of lead. I remember the Prochrono measured 1050fps. It is a little dirty and a real softy. I made this load for my old 1913 Ithaca Flues 20 gauge.