I would look at it as a race SR percentage as you have shown.

If it produces a profit , it has accomplished something positive were backing single horses has not.

I feel the race fav should maybe always included to cover the days where they decide to get up all day

This should minimise the damage if they decided to get up all day , which can & does happen.

I think you will find your best paying races will be Handicap race where many outsiders are know to get up.

Its the short paying races that will kill the idea long term , so try & only target the Handicap races where possible.

Cheers.

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Cheers.