With a spot meter: 400.
With through-lens: 250 or 320.
ISO speeds are for the most part remarkably accurate. If anything, manufacturers try to make neg film so that if there are batch-to-batch variations (which are ever smaller nowadays) then the bias is towards being a little faster than the ISO speed -- a small fraction of a stop. This allows for user error and helps to make sure they reach the stated speed. Thus, an ISO 400 film might be 420 but would rarely be 380.
It is true that ISO standards allow +/- 1/3 stop, so ISO 320 could be sold as ISO 400, but it's extremely unlikely nowadays.
Black and white, where the developer makes an enormous difference, is another matter.
Finally, I'd never heard the idea that 'film' automatically implies negative material. Both negs and trannies are film as far as I am concerned, and indeed as far as everyone I've met is concerned.
Cheers,
R.