The best place to start reading is from the first page of this forum.
Takes nearly a year to read it all but you'll learn everything you need to know.

Ratings as I understood came about from Don Scott who one day was on his way to church and got on the wrong line. Ended up at the track.
From there he built a passion for the races and developed a method to measure horses that were winning in lower grades vs horses in other grades.

A little like Greater Western Sydney in the AFL playing the coming second last playing a top side in the VFL.

Further from that others have developed more ratings systems based on different variables.

Class ratings use variables to determine the best class horse in a certain race
Weight ratings like the ZipForm from the sportsman develop the horses with the measure of how the horse's weight compares to its current official rating.

And so on and so on.

Other ratings like newspapers and unitab I assume take all of these figures into consideration and develop one rating.

Barny by using your database you should run the entire database if your PC can handle it and start filtering your results into categories.
Do a search on William Benter and how he developed his variables.
Sometimes its easier to focus on one track.
I'm considering basing new rules and variable points around different venues/distances.

Good luck