I guess I don't get what you're trying to do. Are you trying to lay out rake walls? If you are snap it out in chalklines on a floor (a diagonal line from the tall end to the short end over however long it is). Pull 16's through, bottom and imaginary top, snap lines through the diagonal and measure your studs. Sorry if this isn't what you're asking or if you already figured it out.
Edit: I figured out this isn't exactly what you are talking about. You probably just drew the darn thing out and built it by now.