Please see my previous comment.

While powder is a part of the "ejecta" helps to produce recoil I doubt very much that a grain or two either way would make a significant difference. In terms of "equation", except for the above consideration of Newton's Second Law I doubt very much that enough is known about the relevant variables of perceived recoil to have valid "factors", much less an "equation"...""crapful" assertions notwithstanding.