I recall a procedure for checking focus shift (effective focal length changing with aperture) that was simple and potentially very precise: place a ruler at a very slight angle to the film plane, and focus on a selected graduation near the center of the frame; make one exposure wide open and one at a small aperture, and after development see which graduation is sharpest. If it is not the selected one, then the groundglass and/or filmholder are out of tolerance; if the small aperture image is sharpest at a mark different from the wide-open one, the lens has focus shift.
Without the right tools, measuring less than the machinist's "half thousandth (0.0005 inch)" is not simple. On the other hand, if a twelve-inch ruler is graduated in sixteenths of an inch, and rotated so the ends are 1/16 inch away from parallel to the film plane, the 1/16 inch markings each represent a depth change of about 0.00065 inch which ought to be close enough. (Bear in mind that the face of the ruler has to be reasonably straight.)
My apologies for the Anglo-centricity of this description; in metric, rotating a 30 cm scale so that the ends are 1.6 mm out of plane will do the same job
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