(20") My yahooing was a little premature. I had a better look today with a ke tester and it turns out I have a central hill. No biggy, I'm not quite finished polishing so I'll try moving the tool in a smidgin to see if it will fix itself.
I bought a laser collimator yesterday to use later but good for testing the haze on the mirror. I would have said the mirror done but laser says no.
It's a bit ironic that I was following Gordon Waite's tutorial and I note he also had a central hill prior to parabolizing.