Hi Sparky,
Try this with your past figures.
With the 1st & 2nd rated only bet them if paying $3.50+
With the 3rd & 4th rated only bet them if $8.50+

This should increase the POT.

2nd fav in race must be $4.00+ otherwise no-bet race.

Maybe make $28 as the cut off.

Cheers.

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Cheers.