TOPIC: How do you calculate free float? I've seen so many different formulae

I've seen many different formulae, such as....
ES of successor - EF of present
ES of successor - EF of present
ES of successor - ES of present
ES of successor - EF of present - 1
ES of successor - ES of present - 1
ES of successor - ES of present - duration of present
ES of successor - EF of present - duration of present

Which is it? And why don't any of the sites agree? Do you use different ones in different circumstances? Do you just try each one until you get a plausible answer (ie not zero if not on critical path and not negative if you're in planning)