I think michaelr is utterly wrong with his suggestion of a "sliding rule". I strongly recommend the only good rule I know from formerly communist Germany: Drive only with 0.00.
For years I practised this rule as a volunteer fire fighter because I never knew when I had to drive for the next mission.
After active service I once violated this self imposed rule, drank a small glass of wine with friends, then another, and after the third I forgot to count. I noticed no impediments when starting to drive home (as a former student, accustomed to heavy ritual consuming alcohol) but saw a red light too late, banged into the rear end of the car before me, and the car stopping behind me was a police car. My salvation was that I respected the speed limit and that the accident happened outside my hometown. Imagine I should have seen a child too late, with 0.2! Thank god nothing worse happened than a tin damage, otherwise I might have killed myself for killing or heavily damaging another person.
I didn't need to serve a sentence in jail. The costs were high. DUI cancels insurance protection in Germany, so I had to pay for the damages of the other car and mine. I actually had to lease a new one for my wife which I mustn't drive - I lost my licence for a year. Now I may apply for a renewal, but before I have to undergo a test wether I am fit physically and psychically for driving. The test is very hard with a lot of flunking.
I also resigned the public office I was elected to out of shame, because I was sentenced.
Over here in Germany this can be done in some cases without trial; you may accept the sentence or demand a trial but it only adds to the costs, so I accepted a payment of 80 days based upon my monthly earnings. So I lost all my savings intended for a necessary house repair.
The costs amounted to about $ 30.000,-. In addition I lost the right to own weapons because the law says I'm not reliable to use them. I'm a fairly good shot and participated even in international challenges with good results, but now I lost my rifle and some pistols which I used for training. I may apply to regain them in five years, and maybe I'll get them back if I keep a clean record.

A cab would have cost me about $ 5,-.

So listen to my words: Stick to the 0.00 rule!

Wilfried