I also made up a chart to carry in my field notebook, all the calculations for each of my lenses out to 2-stops with corrections in both extension factors and EV. The chart makes visible all corrections finer than third-stops. I focused each of my lenses at infinity to find out where on the lens to measure to from the film plane, and I discovered that all my Super Angulons indicated the mark to be at the front surface of the lensboard, and all the Symmars showed the mark to be at the aperture ring - convenient. For the tape measure, I bought a small spring loaded Lufkin metal tape, in millimeters, and it's easily found in a pocket when needed.
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