I outlined DonScott's method in this topic:

http://www.smartgambler.com.au/foru...=3024&forum=2&1

It uses a table converting rating differences to relative chances. From that you can easily verify the relationship is not linear.

As for the number of runners, obviously if you scratch a runner with probability p, you merely divide the remaining probabilities by 1-p so that the total is again 1.