Guessing it's print film rather than slide, if it was me, I'd shoot a roll at box speed and hope/expect the film's latitude and the forgiving nature of the D&P process to give useable results. If I was unlucky and I ended up with very thin negs, I'd dial in whatever overexposure I felt necessary for the next roll.
If slide, I'd use the first as a test film and bracket, using box, +1/2 stop and +1 stop increments. If the results came back still too dark at a full stop of overexposure (from box), I (personally) wouldn't trust the other films but might use them for extra long exposures and play!
As has already been said, in both cases, I wouldn't use them for anything important (unless the test film turned out to give perfect results!)