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With 3 1/2 mm to go I stopped the heavy grinding to see how much of a hole in the centre I had. Did a bit with the 3" ring to tidy it up and then measured it with a 6" spherometer I knocked up. Turns out to be .005" variation over the face of the mirror (between .020 and .025) with 1" untouched around the edge so I'm pretty pleased with that.
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