What I've found with ratings is that when a horse is on the way up a form cycle a normal level of improvement is 1-3 lengths per race so if your 2nd or 3rd rated horse is good odds say $10-20, even though it rates 2-3 lengths behind your top pick it's often a good bet.

With younger horses improvements can be greater even up to 6 lengths from one race to the next, when you have improved fitness combined with less greeness and more racing nous.

So if you are using ratings don't just bet the top pick all the time, consider picks 2,3 and 4 if the odds are good.

If the top rated horse is $3 and the 2nd horse is $15 and there's 2 lengths between them I'll bet the 2nd horse everytime knowing that over time it will be more profitable.