ok
here is what i have so far
level and plum lines on the end of the log
a line paralel to the plumb line to create the mortise and tenon cuts
two lines paralel to the level line for the shoulder and pegs
then these lines are snapped from end to end

the challenge is squaring around the log from line to line but i can see it will be possible to create a M&T with shoulder this way.....

i would appreciate some feedback on this if you are working in the round
-M.