I guess most of us agree it's quite difficult unless impossibile to reach 0.0001" repeatability in Foucault testing. May be it's good time to discuss methods we are using to make the testing to be more accurate.
I'm using tester with a wire instead of a knife. For more accurate reading I never use wire's shadow, but the first diffraction dark line from it (the same can be use with a knife). For marking mirror's zone I use a strip of paper attached to mirror horisontally. The strip has jags on both sites which tops and downs indicate mirror's zones. The paper can be replaced by thin metal strip.
One precaution. One should remember the strip is placed in vertical plane not on mirror's surface. Jags on the strip mark the projection of surface zone's radius on this plane. So it's need to introduce some correction in jag's distance from the center especially for fast mirrors. Also if the strip is wide enough jag's tops and downs should indicate radial distance from the center not the distance from vertical line passing through the center.
Alexander