Don, the advantage I see is you are already at one edge of the mortice/tenon. No need to measure over to define it. What is the advantage of a center snapped line?

Yes, Roger has a point with the uniform brace lengths. But it is not impossible if the live edges are not real wild, it will complicate things, no doubt. Just remember everything is based on the snapped lines, some mapping may be involved, allowing more error to occur.