I didn't design the xBABIP formula so I can't give you an exact answer as to how many season in 100 the line will be within 'x' (my contribution is mostly basic algebra ;)). But let me see if I can't help explain a bit better... The simple fact of the matter (and the point of the exercise, really) is that even given a full season of 650 plate appearances, BABIP does not stabilize sufficiently. You can look at the back of a baseball card for your favorite player and probably pick out 2-3 seasons where he performed way over or way under what he normally did in the rest of his career. The purpose of this exercise is mostly to smooth out those sorts of years from a players career. If you put a gun to my head and made me answer, I'd probably say not to read too much into anything within 10, maybe 15 points of batting average for most players. Rob, thank you for taking the time to answer. So if I understand you right, anything within 10 to 15 points of a players average means his luck was pretty "normal". Anything beyond that means he needs a slumpbuster diet or needs to play the lottery more. Could you do this with a players career to see how lucky/unlucky they are or is that stretching it?