I think it also depends how centred you want the clasp to be.
I have 18cm wrists and have custom straps at 100/95, basically same as the medium one.
The buckle bit itself is centred on my wrist so that the foldover follows the curve of my wrist instead of digging in.
You could maybe get away with either the M or L but could be that your perfect fit is somewhere in between.
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