There are some odd things to look out for. I have a breathalyser, and if I have have had a glass of wine or two at lunch and for some reason (pretty unusual) I want to drive somewhere in the evening, I use it to check that my system is clear of alcohol. Obviously, if it isn't, I don't go - only drive on a zero reading. The other day, having had quite a bit to drink at dinner the previous night, I did as I always do, and checked my blood alcohol level, which in the past has always been zero under those circumstances - I was horrified to find that I was considerably over the Scottish limit (although not that if the rest of the UK). Then the penny dropped - a couple of minutes before taking the reading, I had cleaned my teeth - so don't drive straight after cleaning your teeth! Daft, isn't it!
Mike