Please do not rely on any of the alcohol/blood/breath calculations that you see in this thread. It is not an exact science, and the intake required to get to say 20mg in blood (nb) has so many variables such as body weight, food consumption, type of drink, that there is no safe answer.
Parliament decided to set the limit for pilots at one-quarter that for drivers. I am sure that was not so much scientific as a rule of thumb, but consider what chance there is of BALPA making headlines "Pilots demand right to drink more before they fly".
Different jurisdictions express alcohol measurements in different ways. For the record, the limit in the UK to drive a car is:
35 in breath 80 in blood and 106 in urine. The exact units are different for each. For flying, divide by four.
If the Blessed Danny will excuse just this one commercial link in the interests of educating the readership, there is a cheap and cheerful alcohol meter at
http://tesco.com/electrical/search.a...ode%2Bmatchall
I would not stake my career or my freedom on it, but it can at least give you an idea of where you are.