Micheal,

How about 1 more filter. Only lay those that have a bookies odds of $20 or higher.

Although this means you will will get less as the betfair odds will be longer it will also drop out those selections which won at less then $20. Its also a good complimentary filter to use as confirmation that the don scott markets are not out of whack with reality.

The problem with this filter is you need to do it at race time which does limit the system.