Why isn't there a DMV provision for someone to take a controlled driving test several times under different levels of intoxication. Then you could quantify each person's tolerance level at that point in their life. If someone chose to take this test they could get a tag on their licence that gave their "Adjusted Blood Alcohol Limit."
Because tolerance doesn't stay the same. Really. My fiance's best friend is a massive alcoholic. He finishes at least an 18 pack in an evening and is still highly functioning. And then out of the blue, he can be falling down drunk after 8 beers. I'm not kidding. I know you don't mean that much alcohol, but if tolerance doesn't stay the same, it's better to be safe and disallow driving by anyone with an alcohol level above a certain point.