The legal limit is 35mg per 100ml breath but I was shocked at the difference between people's alchohol levels depending on their size etc etc.
That older guy (quite fat) drank 4 pub red wines and blew a 14mg
There was a woman about 30 (big build but not fat) she drank 8 Malibu & Cokes
and blew a 4mg 
Another guy in his 40's (average build) drank 5 pints of Bitter and blew a 31mg
A young guy (slim) drank 6 pints and blew 41 (I think)
Then there was a young girl who drank 3 double Vodkas and an Alcopop? and blew a 60
There seems to be huge differences going on, and no real yardstick - so be very carefull out there and don't even risk it
(That was a public service broadcast by Welly)
