Hi,
Depending on how you first formulate your ratings my way of thinking is you need a Life to Date average and a LTD Standard Deviation these would become your Base starting points.
Then slight adjustments are made to both of these figures for various Variables, ie a Variable maybe Barrier Position, you then find the average an Std dev for all starts from similiar positions if say you have 5 starts from Todays BP and there have been 20 LTD starts which equates to 25 % then you make that sort of adjustment to both the LTD average an LTD Std Dev.
You do this over an over for the various variables..hope this gives you some food for thought
Cheers