Bought a jig and it seems to work much better than my home-made one. Got out my old Craftsman dado set and cut a few quarter inch ones. Those saws were made BC (before carbide) are OK for regular dados but the cutter blades are not flat. The outer 1" is thinner than the body on each side. So a quarter inch dado in end grain is really a quarter inch with a thin strip of wood left in the center.
Hope triumphed over common sense and I bought a wobble dado blade. Sure enough it wobbles and the dados it makes look like little arched doorways. Not good for box joints.
I sent it back and bought a stacked dado set from HF. Set up the two cutters and ran a few test cuts. This time the cut looked like an "M" with a shallow top. I checked the blades and the carbide inserts were not all ground alike. Some had a flat top and some were canted. It appeared that the teeth were ground by an opium addict with a dremel tool and a diamond blade.
So plan D calls for a real set of dado blades I guess. Still learning at 72 that saving money is not cheap.