Cut rafter tweaking required

[Digmixfill]

I've run into a snag with my cut rafter plan. My ridge beams are level, and the new wall plates are level, but I didn't measure the span distance between the ridge and the walls all the way along. I foolishly assumed everything would be parallel - I really should have known better.

 

One side of the roof is proceeding as expected. 50x150mm rafters at just under 35 degrees with a birdsmouth seat length of 60mm. The side with the progressively diminishing span isn't going to work at the shortest end. The angle changes, and the projection of the rafter at the foot is quite a distance away from the eaves edge.

 

If I put a piece of 22mm timber under the worst offending rafter with the current birdsmouth cut, everything works, but i'd obviously need to use progressively smaller bits of timber on the other rafters.

 

If i reduce the birdsmouth seat length, I can get it to work with the existing wall plate, but i'll be heading toward 30-35mm seat length. What is the accepted minimum seat length? I can only find details for maximum birdsmouth depth.

 

An exaggerated plan view of the roof attached. Red=ridge beam Black=eaves.

 

How would you resolve this?