I regularly drink and drive.Let me qualify that,I have either two pints of 3.6% beer or a half bottle of wine with a full meal over a three hour period. Given my size (6'2 16stone) that most likely means that when I drive I am around half the 80mg limit,leaving a large margin for error.
When I drive in this condition I am aware that I am judgement impaired.This doesn't mean dangerous because on those occasions I drive more slowly and carefully and am aware of my lower limitations.
I have taken part in competitive motorsport.If I took to a racetrack in this condition I would be unable to replicate my cold sober performance.I would either have to settle for two or three seconda a lap slower or I would make an error and spin off.
Flying isn't the same.If you try to drink and fly there is a very good chance that something much worse will happen.20mg is sensible as long as you leave that same margin for error,effectively half a pint or a small glass of wine or a single measure.