Whilst grinding thru "the 3 plate method" for achieving some surface plates, I winged a comparator together to check which were concave or convex: 1/4" thick IKEA Frying pan bottom with three inverted bearing ball points at max 220 mm dia, and a M4 x 0,7 screw in a tight thread at the centre. To that a scale* is fitted, divided into 70 main divisions, (1 /100th of a mm each) and 350 subdivisions (at 2 microns each). Ditched the thought of adding an extra outer vernier scale, as the thread backlash (though within 1/2 a micron) would render it unrepeatable and "pseudo-accurate". How-to: Put on (cleaned) surface, turn wheel clockwise until the whole shebang rotates - back off a bit until all 4 point bearings touch (no more rocking when tapping the disk with your finger, and just when the screeching sound dives in pitch). Harder to describe than to do! Remarkably accurate and repeatable for such a piece of junk! *For the "scale drawing challenged" wanting a protractor/ circle with non-standard (or any positive integer) number of divisions in PDF/ on paper: https://incompetech.com/graphpaper/polar/