They don't stock Saint Crispins last time I was there; they host SC + Ambrosi trunk shows. I would err on the side of 5mm if I am to order again; handwork don't usually have the precision of wood turning machines.
Email them directly. email@example.comSaint Crispins nowadays visits Wingtip @ SF semiannually. It's best to be fitted in person.I ordered mine during his truck show at Leathersoul Beverley Hills but that store was closed.