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How Thick is the Line from a Sharpie?
More relevant is using the ink as an indicator. I was taught by an old school machinist to use dykem or a sharpie and a work light to zero a part to a cutter (esp on a lathe). You color the part of known diameter with ink, then run the cutter in a tiny bit at a time until the ink starts to scratch and finally until the mark left is barely perceptible. If you can get this close you're easily within .0005" which is more than good enough for most machining operations. Then you zero the DRO and you have a starting point (the OD).