With the main floor framed, the second floor joists installed, and much of the subfloor above complete, I have to now calculate the length of the roof rafters. To keep things simple I cut all my joists at the desired roof pitch of 55 degrees, and will simply intall a 2" x 3" 'fascia stop' across the bottom end of all the joists, and then butt the rafter bottoms against this stop. This means no birdsmouth cuts, just a corresponding 55 degree cut at the rafter top to mate with the ridge.
Seeing that I couldnt recall whether 'soh cah toa' (or is it toh sah coa?) was correct, and whether angles had to be in degrees or radians, I chose the most efficient option. I just googled "trigenometry calculator" and got this incredibly simple online calculator. Input two of the values you know, and the other 3 are calculated for you!