Most important probabilities to know?

[paulrq]

Is there any consensus on a basic set of probabilites existing that every texas hold'em player should know? Right now, I'll be honest and say I don't do a lot of mental math at the table, and I'd like to change that--I just dont know where to start. I eyeball the pot and the number of players when I'm, say, chasing a flush or straight draw. I understand the basic difference implied and reverse implied odds. What's next? It just seems there are too many unknowns for an odds calculation to really matter when you're at a table staring at a flush draw flop, and the guy across the table could have anything in the neighborhood of AKs to 99. The books talk about on-the-spot calculations with statements like: "Suppose one of the three players has an 80 percent chance of holding two pair while the other guy has a 60 percent chance of holding trips..." How can odds calculations based on guesses like that be valuable at all? What am I missing here?

Paul