Another angle is to lay the Fav at 20% less than its starting price,
in-running .

Then set a Back bet up to go on once price blows out by 20% of its starting price , in-running, once Lay bet has been matched.

Once both are matched , one has the option to let it ride, so that we make a profit if it wins & break even if it loses.
OR
Hedge the matched bets for an equal profit whether it wins or losses.

Best to use 1400m+ races.
This gives us more time to set the bets.

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Cheers.