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My general rule of thumb after I decided to cool my heels in my late 20s was exactly one drink. That pretty much insured that I wouldn't get pulled over even for suspicion, and everybody knows the less interaction you have with any cop for any reason is a good thing.
Now, even though I rarely drink any more, I won't have even a single drink outside of my home or if I may have someplace to go. The reason for that is simple: I won't step outside of my house without a gun on my hip and the acceptable BAC for somebody in Tennessee with a firearm in their possession is zero.