Hi Phillip,
Honestly, I do this by eye and do not measure it. I find it difficult to get an accurate measurement with something like a feeler gauge. The first thousand or so instruments I setup, I would measure away. After a while I figured out I am more accurate doing it by eye. I have now done enough instruments that it has become second nature and I know exactly how low to go. A side benefit to this is that I can do it extremely fast without it being sloppy. Very important considering the extent of the setup I am doing and the prices of the instruments I am working on.
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