A couple of notes just for discussion:
Note that the rule of thumb cited by Safetypee is NOT conservative when applied to the 747. That illustrates the dangers in relying on rules of thumb when more accurate data are available.
Float is generally accompanied by deceleration, unless the pilot is REALLY gooning it and forgets to reduce thrust. So even though the float itself adds significant stopping distance before touchdown, I don't think the [relatively minor] would be stopping distance once on the ground would be additive.