Shutter speed is 1/200th. highest speed on that shutter is 1/400th. FP3000B has a nice fast dev time of 20sec for 20-23degrees, which is what it was today. But I do take on board what you're saying (which I think is, let the film warm up to room temp before shooting?)
I'm fairly sure that I did try one with ambient exposure, to no avail, but will have another go tomorrow. Light meter! stone the crows, why didn't I think of that? I do have an ancient Minolta Autometer knocking around. Facepalm!
Flash exposure will depend on the flash-to-subject distance, which you have not mentioned. With a strobe, shutter speeds will not affect the exposure you get from the flash, but will affect how much ambient light you pick up.
Try exposing the film with ambient light only, using a meter and allowing for bellows extension.