My friend, Bill Gottesman, and I recently finished writing a note about compound-angle joinery. The math behind the equations for setting up the blade and miter-gauge angles for compound-miter and compound-butt joints is developed. Writing it was our way to figure out compound-angle joinery. There may be simpler ways to do that, but this worked for us. Maybe it will for you too. But you have to have a strong stomach for lots of trigonometry. If you do, here is a link to the note, titled Compound-Angle Joinery.
-- Don Snyder (38.6N, 90.3W)