Just to update. Thanks again everyone.
I planed a flat spot in the centre of the beam and stuck a level on it and used that to level up the beam. Then at each end I shimmed a square to level and used a second square to mark out the bottom of the ideal timber (bottom of the tenon in this case) and the bottom of the joist pockets on each side. Then I marked two positions on the top for the width of the tenon. I repeated this at both ends and used a chalkline to join the dots.
One mistake that I made on the first end that I cut was to not join the "ideal timber" lines around the end of the timber before cutting the cheeks of the tenon. Once cut, transferring the line around to cut the shoulder got a lot harder.
The joist pockets have all been cut with a router to the face (so will have some degree of twist in them) and will be tuned to vertical once the beam is in place. The centre joist pockets which incorporate a mortise for a tusk tenon have already been trued up to the flat spot in the timber to ensure that the frame can be put together without too much fettling.
I'm one mortise and two tenons away from lifting the last section of the frame, and I'm no longer scared about what to expect from the ridge beams. Thanks again.
