You can go to Wal-Mart and by a triangle square call a "Rafter Layout Square". It will have the directions with it on how to cut rafters. It also comes with table rafter sizes. Costs about $3.00.
You can do what I did and draw out a full size rafter lay out right on your shed floor. You draw out a full size trestle or two rafters right on the floor and then block it in for a jig. Then you can make up trestles fast and easy. If you want to go with a rafter then just make two up for a test fit. When you have what you want. Use the first as a template for the rest.