Rates of elimination of alcohol will vary between individuals, depend on the amount of food in the stomach and no doubt other things too. If an airline pilot on tour is having to do mental arithmetic to figure out whether they'll be fit to fly at 6 am the next morning then ergo they are drinking too much. Forget the maths, I can't see how you can justify drinking more than one unit the night before flying. Quite apart from anything else, flying with a hangover isn't to be recommended either.
QDM