"Rent to Own" advice needed please...

Hi all,
am needing some advice from someone out there with some Rent/Own's under there belt:

senario 1 - contacted by an owner who is open for me to lease/option there 3 br duplex in 6 months (they have it leased till then) time. So I can create a back to back (or sandwich) lease option for 2 years with an exercise price of 330-335k owners estimate current value of 310k, a quick check of whats online shows that this seems pretty accurate. So my questions are;

1. Whats the possible/best way to structure the deal?

2. How do I know the numbers will work? (I expect to make profit more in cash flow rather than the backend profit. Infact cashflow is my immediate need. After a chat with the seller I think her need is security)

The deal is in a slightly below average suburb in Perth.
Feel free to share your much valued opinion here or sms me on 0419321733
and I will happily call you (even if you have 5 minutes to chat). Thankyou very much :D