Just a bit for you.

I work my POT at the end of each day, you cant work out POT when you win, when you loose but at the end of a "session" be it day/week/month/year.

I personally wait till close of play each day, either look on the paper columns or in TAB bet column which tells you how much youve spent and how many bets youve had (the only good thing about NSW TAB), add up ALL BETS, then add ALL WINS (if any) and as the others say on reply to your question, the differance between BETS OUT AND WINS IN IS PROFIT, DIVIDE PROFIT AMOUNT$ BY BETS OUT AMOUNT IN $ and wallahhh, POT in %. At the end of each month i add up the 30 days or so and get a total then divide that by 30, average POT.

IE,
Yesterday,
BET OUT 910 units
RETURN 1137

PROFIT 227 units
Therefore 227/910=24 %POT
So as Bhagwan says, to make 1000 units POT you need to spend 10,000 units at 10 % profit, so it depends on many things !

Hope ive helped
Cheers Dingoboy