Originally Posted by Shaun

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%