Here's one way to bet the top 5 rated, using estimated prices.

Bet 1.0 unit on the unders.

Bet 1.3 units on the overs.

Try & target races where the 3rd Fav in the live market, is 6.20+
This generally helps to avoid being confronted with constant short paying results, thus helping in the process towards showing a profit overall.

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Cheers.