My 1 micron/ div indicator sitting in the middle of the Spherical Comparator (bottom of a former frying pan, hence the faded Teflon), together with 3 M4 x 0.7 mm SHCS bolts w a silver soldered 1/10" ball bearing as a bearing against the surface to be measured. All this boldly winged together thru ideas from: http://bobmay.astronomy.net/spherometer/spherometer.htm. Variable radii (110, 82,5 & 55 mm) to get into the corners. Now this one measures the absolute deviations from flat within the chosen radius. Just as my homegrown 3 plate method (shown elsewhere in this folder) was finished, Providence had me find a free*, unbranded & uncertified surface plate the other week - measured > +/- 1 micron (@ 220 mm dia) along its diagonals. Free "Tool room quality" (less than 1,38 mu) plate for my walk-in closet workshop. * Its odd dimensions 413 x 266 x 72 mm not available thru ordinary channels, a new one with equal properties would set me back £ 300 + freight cost for this 25 kg Diabase slab... Cheap headstone-to-be for yours truly, perhaps.