I test all my shutters in advance with a Calumet electronic tester. This involves multiple readings for sake of predictability at any given speed, which is more important than nominal speed accuracy. Any slight deviation from the norm can be put on a lensboard label for sake of fussy applications like color chrome film. Every one of my Copal shutters of any size or vintage has been well within a third stop at every typically used speed, and generally even more accurate than that. But not a single one of those shutters, even brand new, was even remotely close to accurate at the very highest speed (typically 1/500th). I once had a Compur 3 which was spot on over the entire range, but disliked it for other reasons. Studio photographers were more concerned with flash speed synch, while people like me don't give a damn about the top speeds anyway, at least for view camera use.