I'm not a math major, so there's every possibility I got it wrong.
The way I figured it, if your average post rate is two posts per day, and you joined in 2000, it's 2000/2 = 1000. If you joined in 2012, it's 2012/2 = 1006. Since it's an inverse relationship, the lower the ratio, the greater the supremusness.
But now that I think about it, it doesn't really work that well, since you could have somebody who joined in 2000 post twice a day, which comes out to 1000, and then somebody who joined in 2012 post four times a day, which comes out to 503.
The whole idea is to highlight the fact that lots of posts per day + recent join date = less supremusness. There's got to be a way of expressing that mathematically.
You know you're an audiophile when you suck at math.