First Possible Lease/Option to Purchase Deal

[Ben Smith]

Hello,

I am writing an offer to a seller that I think is a great candidate for a lease to purchase. He's been in the house for 10 years, his son was born there, etc, etc. He got a divorce and lost his job a few years ago and couldn't get caught up but hates the thought of not being in the house. 

I would love to keep this guy in his house and make some cash along the way.

I'm having trouble coming up with a number for his purchase option. Do you add a certain percentage to the value to account for appreciation?

I'm offering $70k. It's probably worth closer to $85k (with easy cosmetic work: paint, power wash, etc.) I just wonder what is fair and profitable for a purchase option price at the end of a 3 year lease.

Thank you in advance!

Ben