The box ISO. But as I said, just shoot it at the box ISO first and see how it comes back. The reason I said shooting 1/3rd to 1/2 sop lower is because negative film tolerates overexposure better than underexposure, so when in doubt, overexpose. But still, it is best to shoot the film at the box rating if you are sending it to a lab, because they are developing based on that speed (and they go through a lot of trouble to figure all that out in testing!...or at least they should!).