Yes that is correct moeee but when you have an animal which weighs, 600-700kg (correct if I am wrong there but ballpark) how much difference is 1 extra kg going to make.

The book is an interesting enough read and there is some mathematics in it which may take a little time to grasp, but it's not War and Peace.

It is basically taking a time that a horse recorded for a distance in a previous run, usually the last or 2nd last run. Converting it to todays distance with an adjustment for weight, up or down and Hey Presto.