Have half a litre then. Speaking as someone from the only country that legally mandates beer in both pints (on draught) and metric (in bottles), I prefer a (UK, 20oz) pint if I'm drinking, because it's larger and I'm thirsty. For every other reason though litres are easier to reason about. The US 16 oz pint doesn't even have the benefit of being larger!
But usually in British pubs, beer is served in glasses that are a pint up to the brim, so if you are served beer with any head at all it’s probably closer to half a litre than a pint.
It's physically impossible to measure exactly 500.(0) mL [500.0000000000… mL with infinite number of zeroes] of any liquid with absolute accuracy. It's just the law of metrology. There always will be an error, you will always be able to create better tools to add extra decimal places to your measurement and find more error. The question is how much of error you can tolerate in any given situation.
The error has nothing to do with units of measurements chosen. You cannot measure exactly 1.(0) gallon [1.0000000000… gallon with infinite number of zeroes] of liquid with absolute accuracy either.
In engineering this would be resolved with something like 500 ± 10 mL.
In consumer goods there are either regulations or you get, say, 505 mL so you have nothing to complain about. And you would never be able to tell the difference.
