It's hard to say Ron. Personally, i dont like to automate the base rating too much. I look at the 'good' ratings a horse does say in its last 4/5 runs. Then i look for a few things:

Can a horse be expecetd to improve at all (lightly raced, improving ratings)? Has it done a peak performance and now should revert to its old lesser form? Is a horse rating well below its best on going/distances that should suit? Has a dry tracker been hitting wet tracks recently?

That sort of stuff.