Just do it the way you would for win betting, What ever you payout is your outlay and what ever you win is your return.
For example
If i lay 100 runners @$100 = $10,000 this is my return.
If i paid out in liabilities $8,000 this is my payout.
Results $10,000-$8,000 = Profit $2,000
POT $8,000/$2,000 = 25%
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