There is only one rule to follow on TSM. And no, it's NOT admitting that you are powerless, or taking a moral inventory, or turning yourself over to a higher power, or making amends to everyone you ever harmed, or spending the rest of your free time dedicated to spiritual growth and your awakening, not to mention spending the majority of your free time in some dingy room with bad coffee and chain smokers for the rest of your life. No, none of that. Just take the pill an hour before you drink -- that's it.
In light of the other "alternatives", taking your naltrexone in advance isn't too much to ask. I'm not spiritual and I'm not religious, but I agree that religiously taking your naltrexone one hour before drinking is a good religion to live by.
All that being said, one slip up will not have a major impact on the de-addiction process. But don't do it again, you are re-addicting yourself any time you drink without naltrexone at a hyper-fast pace because of upregulation. If you don't know what that is, look it up on page 121 of Eskapa's book.
Take it easy.
Nick