Some food for thought: I do not like to reduce the depth to fit the joining member. Reason being, is that I am under the philosphy that your timber is only as strong as its weakest section. I wouldn't want to create a possible shear point there. I would just upsize accordingly.

I recently had to wrap my head around something like you are doing now. It was a 24' X 24' 4 hip structure with a 3/4 pitch instead of your 1/3 pitch. It was a doozy. One thing that I learned though from it and recommend to you is that you find your longest rafter (excluding principals) and determine what cross section that needs to be and work back to the principles from there. A 24' triangular roof plane is a lot to fill in.

I'd be happy to show you an easy way to find the angles for the top of your jack rafters, as well as the backing cut of your hip, when you get to that if you like