Primary residence: Lease with Option to Buy?

I've searched and everything I've seen is applicable to deadbeats with bad credit or landlords who want to screw deadbeats with bad credit. I fall into neither category.

We are moving to the Chicago area and are planning to rent a SFH for a year or two so we don't have to fully commit to the area before we get comfortable here. Lease to own calculation shows that we're at a 6-7 year payback period for a purchase (as against renting). If we do wind up committing to the area, we'll stay here for a while.

We've found a neighborhood we like and a house for rent (or for sale) in that neighborhood that we also like. Owner is open to lease with option to buy. We'd consider this if the option fee was reasonably small and if we could get a reasonable percentage of the rent applied to a reasonable purchase price. Basically I'm looking to recoup some of the rental payments if I can do it for an overall cost less than that of buying then selling the property before the lease-to-own payback period.

I have a good understanding of the legal arrangement but am looking for help in calculating the financials associated with such a deal.

Anyone have a pointer to an online calculator, or a spreadsheet they've used to look at such a deal before?

TIA for any help, and any general advice about such a deal.