Personally, I think it should not be so cut and dry, for example, it should be scientifically formulated based upon the driver’s listed height and weight (as noted on their license—which is legally required to be accurate within small degree of error), should account for distinct tolerances between male and female, should account for the person being of drinking age (e.g., under 21-years of age with a BAC of .02, they would be deemed legally intoxicated), and it should be required that the driver exhibits one or more signs of intoxication that would attribute to diminished motor-skills or physical reaction (excluding from this requirement—as a singular indication: scent of alcohol, bloodshot eyes, verbal admissions, flushed skin, unkempt appearance, etc.)
* While on the issue, I also think that it should perfectly lawful for people to drink alcohol while driving, so long as the driver is able of maintaining their BAC below the legal limit. If you can handle a coffee, pop, or juice while driving you can just as well handle a can of beer, a sip of vino, or a swig of Black Label.