We cut a rafter to what Stephen's math said it should be, then tried it out and adjusted. We used that rafter as a pattern, and cut the rafters.
The rafters were huge, and a nightmare to get into place with only two of us and no special equipment. We cut stretcher boards to go between the rafters to give a ceiling height of 10' in the Great Room (20' of the total 32' length of building,) and the loft boards to give a ceiling height of 7'6" above the bathroom and kitchen (remaining 12' of the length of building.) The rafters kept kicking the sidewalls out of square, and we added considerably more temporary supports.