At the risk of sounding very boring, a good rule of thumb (at least in the UK anyway) is just don't drink if you're planning on driving.
I know some people get away with it, I just don't think it's worth risking it for the sake of a couple of drinks.
A clever device and app nonetheless - good luck to it, as I suppose it could benefit many!
I operate under that method however, the reason I have bought a small breathalyser device is for the morning after. It's common at the moment to drive to my friends house, sleep there or in a tent and drive back in the morning.
A lot of people don't realise that you can be over the limit in the morning and quite easily too.