So the plywood "straightedge" wasn't so straight.

So it would seem. I checked it with my straigt edge prior to tightening the
7 screws down, but there was obviously *some* error.... :-(


You might do better using a tablesaw. Screw the plywood strip so it
overhangs
the opposite edge, and bear against the TS fence with it. This way, the
impact
of small variations in the plywood will be reduced by the long fence.

--
Art

I thought about this and it should work in theory. Here's the problem I ran
into in practice.
First, the fence would have to be over 3 1/2" high for the plywood to bear
against it. No problem, fasten a tall subfence to it, right? Well yes, but
now you have the problem of the mass of the timber pressing sideways against
the top of the subfence which is fastened to a short TS fence. The leverage
of all that mass will make the subfence flex away from the blade, I'm guess
substantially. Secondly, I only have a 10" blade for the table saw which
has a max depth of cut of 3 1/4" while the boards are a *strong* 3 1/2. So
the blade wouldn't even make a through cut, which would concern me from a
safety issue with all that mass against a comparatively flimsy fence.

jc