Unless I'm not understanding the question, you may not be able to do this safely.
The #12 shot is of no consequence as shotgun loads are dictated by shot weight not size.
Attempting to arrive at an unpublished shotgun load is particularly dangerous mixing different powders, their rates and wad columns, even with different hulls.
At a glance, the lightest powder charge and lowest velocity I seen for 7/8 ounce loads in 12ga and 20ga was about 1180fps. I'm sure this is going to be too powerful in the confines of a basement.
If you are getting fair results with the 22 snake shot, you should good to great results with 38 or 44 shot shells.