In my week long introductory timber framing class last week, they called out method square rule. The project timbers we used were very nice, planed, and accurate to within 1/16th or better. The also had very little crown (bow?). We marked our outside face and the top as our reference faces and made all measurements from those faces. This makes sense so that you are basically using the same face of all the timbers of your bent as a reference plan and therefore all measurements being made from this same plane will assure that the mortise and tenons align to this plane (and the top reference plan. Then as long as you make the other two faces of the mortice or tennon square and to the correct dimension, it will all fit together correctly. From reading this post, it sounds like mill rule and edge rule are the same thing just a different name. Is that correct?