I'm just trying to get my head around overs and unders. At the moment I don't see the logic.

If a horse has $2.00 fixed odds in Race 1

On racing and sports the don scott rating might be $1.50, hence a overlay.
But another set of ratings might have this at 2.50 which is an underlay.

What set of ratings are correct? I would assume by being able to see over history - comparing both the strike rate of all $2.00 rated horses on don scott and the $2.00 rated chances on the other ratings.

Should you not see a 50% strike long term give or take.

If the strike rate over 50,000 bets (example) in racing and sports for $1.50 horses is 40%, then the price is not $1.50 but it should be set at $2.50.

What is the point to finding value in ratings when you are really underbetting in thinking its an overbet.

I suppose it is really difficult to measure as we don't know the true value of a horse and its chances. We can assume on history only.