I've seen it often repeated here. From what I've been able to find, it was originally from brewsupplies.com, which doesn't appear to be anything anymore. I wouldn't take it as gospel, since from my understanding, hop varieties, gravity, etc. also play a role, which this is obviously not taking into account. With aroma/flameout additions, if you remove them during cooldown and the time it takes to chill, etc. would also probably play a role. But as a general guide, I'm sure it's an okay.