One way that can work is...
. Target fields of 8-9 runners only

.1200m & less

.Target no more than 5 runners.

.Multiply the pre-post price by .90
e.g. $6.00 pre-post x .90 = $5.40

.Bet all the overs by dividing your price into ones base amount .
e.g. Base amount $10 divided by say $5.40 = O/L $1.85

. Bet the ones that are at least 1pt. greater than your price.

Use Betfair Trader Pro, its free (Google RacingTraders) to use these fractional amounts on Betfair.

Cheers.

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Cheers.