The basic calculation for a man is [7* Ethanol in the body (in ounces) / Weight (in pounds) = BrAC in gms/210 liters of breath.] To calculate the amount of alcohol in the body, add up the total ethanol consumed and subtract .5 ounces for each hour since consumption. (3 ounces of 80 proof liquor would constitute (3*(.80 / 2)) or 1.2 ounces of pure alcohol.
To calculate the BrAC for a women, do the calculation for a man and multiply by 1.1 times.  The average women’s body has more body fat and less water per pound than the average man’s body. Therefore, the same amount of alcohol consumed will be more concentrated in a women’s body who weighed the same as a man.
As you know from the About Alcohol section of this website, the dynamics of alcohol in the human body can be quite complicated and the above formula can only produce a very rough estimate.