[polarbear]

There is a famous baseball sabermetric formula that covers this, called the Pythagorean Projection. However, the balls aren't distributed exactly "randomly", as baseball scoring is peculiar.

Anyways, someone (I believe Bill James) in the '80s found out that a good approximation was:

Team Winning Percentage = (runs scored^2) / (runs scored^2 + runs allowed^2).

Later on the formula got refined, with the powers being a number around 1.8 (I don't remember the exact number), and everything else the same. The small change doesn't change the formula's win% by much.