No one can give you a definitive answer to your question since your own, individual E.I. will depend on your equipment, metering techniques, etc. Unless you test yourself to find your best E.I., you'll never really know.
That said, rating your film 2/3 stop slower than box speed is usually a good starting point for testing. If I had to work with untested film, I'd likely expose it there; a bit of overexposure is no problem, but underexposure is.
As far as using roll film in situations with substantially different subject brightness ranges goes, you really just have to be very careful metering and compensating for very contrasty situations. If you are spot metering and basing your exposure on a shadow value, then there are no problems, but if you are using an in-camera averaging/matrix meter, the camera will tend to underexpose contrasty scenes. To compensate, add a stop of extra exposure with your exposure compensation or whatever.
Best,
Doremus