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@gbj_tester
It's a bit more complicated than that, since the tyre is roughly a circular cross section tangential to the edge of the bead hook, which has a diameter about 10mm more than the bead seat. I find 626 + twice the tyre width is closer, but that's for thin tread race tyres; you could easily add another 2mm for something with a thicker tread band and breaker even on all purpose tyres, and some touring tyres are more like +10mm.