What I do is this, when working out a jockey challenge market:

In each race, award points from 6 to 1, in order of favouritism, so 6 for the fave, 5 second fave etc, AND give each rider a point just for having a ride. There is room for leeway, like if one of the faves will be Might and Power in the QE Stakes, ( ie 1/10 odds) you can give him more than six. The six through one is a guide to the "normal" race, adjust if it's an abnormal one.

Add em up.

Divide any riders points by the total, multiply by 100, that's their percentage chance.

It's crude, but it's pretty accurate.

Duritz.