Hi Chrome,

If you rate each horses individual performance an make no adjustments for trouble or whatever and then find the Standard Deviation of its performances then do this for hundreds of horses from the Best to the Worst the youngest to the oldest you will find the answers your seeking.

If you use the Probability function within Excel an try manipulating the Stdev an check the effect it has on the probabilities I'm sure you will agree the importance isn't on the avg rating figure but its to be found within the stdev figure.

The main focus of ones attention should be on how to tighten or widen each horses Stdev.
Then find Variables/Factors that allow you to make minor adjustments to the Stdev based on either the individuals past performances or by analysis of all runners for that same Factor eg Horses starting from say inside barriers over X distance may have a very tight Stdev based just on those past performances, so a Horse starting from that position today may need to have slight adjustment to its Stdev based on this analysis, or say horses returning from a very lenghty spell may have a bigger/wider stdev so we might make that adjustment then to that same horse.
You can also use it to isolate races which have a tighter overall average stdev which means a more stable result, an this is what in fact will happen. A step further is to find in those above races where your top horses have a bigger advantage over its rivals, then your Golden