I can't remember what thread it was now but I can explain quickly.

Basically I do my ratings by applying a score to each horse, and using the scores I estimate my odds for each horse. Once i've got these odds I sort to find my top rated horse, 2nd rated and so on.

If the odds of my top rated horse are less than 75% of the 2nd rated horse, then its a play. If the 2nd rated horse is less than 75% of the 3rd rated horses odds, then the top 2 raters are a play.

I hope that makes sense, if not I can provide examples.