I don't know the specifics about Jamin's situation but I do get the feeling over the years that there's a problem here.
I'm going to have to agree with Flaggie here (first time ever) in that if you're scheduling/planning drinking as a reward then there's definitely an underlying problem.
And, speaking from experience though give me some rope on this: you're lying to yourself if you think you'll keep it to just one.
And then once you kill that 6-pack that you were only suppose to have one of, instead of admitting a mistake and re-starting the effort, it'll be another 6-pack the next night (Thursday night is practically Friday night anyways). Then it's the weekend - all bets off ...and then the 1beer/50miles is never heard of or seen again. There are people that might not understand this: just-one-drink doesn't work if you have the propensities of a problem - maybe it works if you are going out with friends who aren't truly non-drinkers (I personally know only ~2 people like this, Flaggie makes 3) but if you're buying some beer to bring home and have alone, it wouldn't stop at one.
If you haven't already, look up some videos on Youtube about people dealing with alcohol/alcoholism. Sobriety doesn't sound like fun (I'd never go completely sober) but listening to some stories of the mental tactics in some of these videos could help.
My strategy, which admittedly doesn't hold all the time, is to only have a drink (6-pack) on evenings when I have a planned day off from training on the next day (usually every 21st day) and holidays when with family/friends and after some races (<--but those can be the most dangerous). The "no drink if running tomorrow" rule helps me when I have the urge to think about the next day and how that run/session is more important to me than drinking at that moment.
I understand that my strategy is a form of "scheduling drinking" but it's more fixed and standardized than a mileage standard - so maybe that makes a difference. Hope this helps.
-Krispy