I finally got off my lazy hind and built a Focault tester.
The mirror holder was the only new thing I had to build. I had a slit and knife edge ready from a previous attempt.
After gazing at the myriad plywood pieces lying around in my balcony, I hit upon an innovative design for the mirror holder. The triangle pieces cut-off for the Dobsonian cradle came in very handy.
The actual Focault test was fun too. Here is a view of the image, with the 50% region at null.
Both the turned-down edge and the central hill don't bother me too much. I have some mirron-on-top polishing to do which should even out those areas.
What really bothers me is the number of scratch marks my mirror is accumulating due to prolonged polishing :(
Anywho, the focault test is a good guide for knowing when to stop polishing!