Read the stuff in V.M. Starr's article found at Bob Spenser's Black Powder Notebook.
[url]
http://members.aye.net/~bspen/index.htm[/url]
He suggest starting with 2 drams( 55 grains) with 3/4 oz. of shot, and working up from there. I would not load more than 2 3/4 drams of powder in any 20 gauge gun, because, as Starr says, you are just wasting powder and shot.