4 test cuts later, and I have a joint that works. I started with a -0.125 mm clearance on the mortice, moved to -0.250, then tried -0.5 mm. The last works, but is now a bit loose. I’ll try again tomorrow with -0.375.
I also completely redesigned the joint. The other one was adapted from my Raven Desk joints, which were cut with a 1/4″ cutter. This time I have to use a half inch cutter because of the depth of cut. This latest joint is based on circles with a 15 mm diameter, assuring lots of clearance for the tool.
One of the cuts dislodged the work piece in the fixture and I hit the Stop button as fast as I could. After that I ran screws into the workpiece from both sides, in addition to the Bessey clamps. The screw hole areas will probably be cut out anyway, both top and bottom.
I just realised that if I cut from the outer side instead, I can do a half-blind mortice and not have that gap at the joint., OK, back to “the drawing board” ie. VCarve tomorrow to try that approach.