Something that I read on this forum, and is much easier for me, is to simply take the total extension from the center of the lens (non-tele) when focused and divide it by the absolute aperture size (lens focal length divided by f/stop). This gives the "corrected" aperture, which you can use for exposure calculation, or if you want to shoot at a given aperture, you can easily convert in the opposite direction.
IMO, faffing about with the magnification is overly complicated, but that's just me. Unless you have to have a specific magnification for scientific applications or something.
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