This article and video presentation will give you good information on cutting and fitting rafters. Reviews calculators and iPhone apps to do the math.
http://www.thisiscarpentry.com/2012/11/09/common-rafter-framing/ (http://www.thisiscarpentry.com/2012/11/09/common-rafter-framing/)
(http://www.thisiscarpentry.com/wp-content/uploads/2012/09/Adjusted-Run-Diagram-1-e1348695691694.jpg)
Just in time for a little shed, thanks John!