Just after some clarification on calculating the profit on a fictional set of bets. So take the following numbers taken from betting BFSP based on backing them for 1 unit, and tell me have I got my working correct.

Bets - 100
Wins - 20
Losers - 80
Return 85
SR - 20%

So working out the lay profit, are the below figures correct?

-80 (lay winners) minus 5% would be your total lay return, so 76 unit return
-your payout would be 85 minus the number of selections so 65 units paid out

therefore your profit if you were to lay those would be 11 units, is this correct?

No. Lays are worked out completely differently.

Lays are based on liability not the opposing bet size. You would need to change your liability to get the same results and then it would affect SP and the figures would turn to a loss.

I need all 100 odds for the bets to help you work it out properly.

Okay, so I understand that if you were backing to liability it changes dramatically. In the above example, if you are just letting someone back each selection for 1 unit regardless of liability and exposure to bank, would the figures then be approx. correct?

My thinking was that they have backed 20 winners, which including there 1 unit stake has returned 85 units total to them, this would mean I have paid them 65 units of my own bank. Therefore I am down 65 units on paying out the liability to those bets, but then have taken there 1 unit on the other 80 bets, therefore 80 units up before commission.

You can't lay the way you want to on betfair SP. You can only lay to a liability. That means if you want to make it $1 then for a $100 shot you are risking $99 and for a $500 you need to risk $499. The problem is the more money you put into SP the further the odds move out.

Anyway as per your example assuming you changed your stake and were miraculous in getting your $1 lay exact then you would have the following outcome:

You won on 80 of these. Your assuming 5% commission but it will probably be higher in some aus markets. I'll use 5% as that is what you used.

80*0.95 = $76 in
You paid out $85 for the winnings and got in $20 for those so your outlay was $65. 76 in minus 65 out and you get a profit of $11

Your only problem is going to be depending on the odds you are laying you are going to push out the SP odds slightly which will eat into at least half of that profit.

Generally laying is done by liability (minimum $30 at SP) which is very similar to backing for a fixed stake.

Thanks for the clearing up, I did not realize you could only bet SP to liability.

Glad to see I am on the right track in regards to basics though.