I glued the plywood down to the bench along side the neck so that it was all flat, tight and immobile.
so, now that I have the router aligned, and the rails mounted on the 3/4" plywood I just need it to stop at the right place so I don't cut through the end of the neck.
the router base is a 6" diameter. the bit is a 1/4" bit. so, the edge of the bit is 2 7/8" from the edge of the router
the end of the groove is 7/16" from the end of the neck,
so math, and I set the router where I want it stopped. Then I put 2 bolts in the other groove on the clamp tracks:
same thing on the other end, and now I have stops that keep the length of the groove exact.