I'm guessing but I think it might be Pan F+ ??
If so, it has a box speed of ISO 50 but I've tended to shoot it at ISO 32 in the past - for no good reason other than I was always told it was best to shoot B&W film half - to a stop - over the box speed.
It's years since I shot it but it is wonderful film - but, again, I'm told that the emulsion is a little soft and prone to marking and it needs a bit of TLC when processing.