If one wishes to get really tech-nickle about the whole matter, ISO is key in determining a film's key stop which is the square root of a film's speed rating (ie. square root of 400 is 20, key stop = f/22). Along with determining a starting shutter speed going from your meter reading of a middle value (1/(middle reading's candles per square foot) gives you a base exposure from which to begin to determine where you want your negative to go.

Of course, that's REALLY tech-nickle.