Here's what I posted 12 years (!) ago for coming up with semi-reasonable probability numbers for a single race using any rating. (Assuming you don't know anything about the rating or haven't done a study of it anyway.)
See my post#7:
http://www.paceadvantage.com/forum/s...ad.php?t=11081
Then there is the convoluted "tennis tournament" method from the old book "Beating the Races with a Computer" (1980). (Which is mainly a curiosity of how you might do such a study with the computer power of the late 70s) See this thread and my post#17:
http://www.paceadvantage.com/forum/s...ad.php?t=60199
I'm not reposting it all here since the context of those other threads helps. (The bottom of the second thread has links to even more threads!)