Ozracetools have a handy program for resetting the book percentage.

So
Punch in all the Bookies prices which will come to approx 115-130% on opening prices
Then make the program set it to your percentage, say 100%

This 100% tries to reflect the horse's true chance of winning, if the bookies prices are accurate

Then maybe use Betfair to get the overs for win betting .

Now if we find ourselves getting bit, profit wise, along the way, it tells us that the general public is more accurate at accessing the markets chance of winning.

In that case , one maybe better off going for the unders.
e.g. Adjusted Bookies accessed price $4.50 then only bet it if its under this price on Betfair.

That's the general idea .

One will usually find that it swings from unders one day to overs getting up the next day, so one has to try & view these things over a number of days to see which is the more profitable.

The late great Don Scott did extremely well at doing his own ratings & betting the overs.
He would bet his ratings to price set to a to a fixed return.
E.g. His accessed price say $3.50 into his Return figure of say $1000
O/L $286
The horse he is targeting could actually be 7.00 with the Bookmakers.
7.00 x Bet $286 = Ret $2002

He would usually bet between 2-5 runners a race.

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Cheers.