It is not clear to me what is the right way to layout mortises and tenons in my situation. I need to insert a cross in a frame in which the rails are not flash with the stiles but 4 millimetres offset.
In my opinion I should place a piece of wood on the rail and between the stiles and plane it flash with the stiles to compensate the difference, and then mark as usual. But maybe there is a better idea.
While reading your comments I think I found the solution. I will take a piece of straight wood bar that is longer than the distance between the stiles, rest it along the stile, set the gauge and mark the mortises registering the stock of the gauge on this additional piece. After that I will place the same bar across the stiles and right above the rail and mark the mortise the rail.
I feel the photos will make it clear what I mean.