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To me "working distance" in macro is measured from the front edge of the macro lens, rather than from the film plane which is normally used for the distance settings on the lens' focus scale.

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That is correct for you - but everyone else counts working distance as being from the 'film' plane to the subject. It's that distance that's used in all the formulae.

It's also because of the above that the working distance (by your definition) isn't as much more as one might think when swapping from, say, a 100mm lens to a 200mm. The extra length of the longer lens eats into 'your' working distance but not into 'mine'.

Hope that's clear

MickLL