Trial and error. If you expose it so the shadows in the scene just have detail (i.e. Zone metering, film calibration, etc.), you will likely be exposing it around 800 to 1200 EI. Any exposure less than that will likely eliminate detail in the shadows, but you may have to do that to hand-hold a long lens. Low light film photography is difficult. If it were easy all the digicam people would be switching to P3200.
Since the film has such incredible latitude in exposure (i.e. you can get printable negatives with an EI anywhere from 400 to 64,000) this is how I use it. I set the lens wide open and set the shutter for the slowest speed I think I can hand-hold.
The negatives will be varying densities depending on the lighting. Some will be impossible to print because of no shadow detail, but it is-what-it-is.