Rs = Runs Scored
Ra = Runs allowed
x = ~1.8
Exp Pct = Rs^x / (Rs^x + Ra^x)
This is also called pathagorean win percentage because the formula is similar to that for pythagorean trianagles when x = 2. I think WIS uses an x of ~1.8.
In WIS, if you are managing your team well, by season's end your actual pct should exceed your expected.