Bump, I'm quite interested in this so I'll take a stab hoping for correction.
I didn't follow all well but am assuming 11 rafters and the thrust at each post top is 2611 lbs. The plate needs to take this without rolling off.
Following Mike's comment, the bending moment in the post at the tie mortise would be 2,611 lbs x 2'= 5222 ft-lbs.
I looked at the post as a beam overhanging a support (the tie), with a point load on the end (the thrust). The "beam", our posts section at the point of maximum bending, is the post less mortise. I looked at it using this calc I had;
http://windyhilllogworks.com/Calcs/pointloadcantilever.htmI'm not sure that is entirely correct in that there is also an axial load down the post as well as the thrust's bending load...but in that area the tie provides lateral support.