Like I said, test and test again. That's the only indicator you have when dealing with this. 18, even after drinking, is way too high really and if you think you will plummet as a result, you should ask yourself a few questions as to whether you want to run the risk on the high or low side, and what you can do to manage it!
I've been there with the drinking and I know how hard it is to manage the downside risk, but the upside risk while not immediate, is just as bad.