The only thing that is critical is the actual measuring of the position of the KE along the direction of the axis of the mirror. That is the only measuring device on the tester and if you build it to the design, it will work.
The remainder of the tester just needs to operate smoothly enough to be controlable in the cutting off of the beam of light returning from the mirror.
The Foucault test is really a simple test of measuring the radius of curvature of a reflective surface by the knowledge that light emitting from the center of a sphere will return to the center of the sphere and form an image.
The only thing that is really important to measure on the test is the change of distance for the various zones to be able to do the calculations for the difference between a parabola and a sphere. The actual full distance is relatively unimportant with 1/2" being as accurate as needed to be measured.