@Brandon Kessler I am currently working on a lease back situation with an owner occupied duplex now. I used an LOI with an attached comp list of sales and rent data from the area. Using the LOI and comps helped us both feel good about the details for both agreements before either of them were drafted up. As part of the LOI process, we agreed to put high level rental details into the purchase agreement to confirm the sellers option to stay. This included language something around the lines of "At time of closing, Seller has the option to lease back the currently occupied unit for $x,xxx a month for 12 months, WSG excluded" (paraphrased)
As we near closing, we'll finalize the actual, legally binding rental agreement for one of the units and I'll inherit their current tenants in the second unit.
This doesn't help you on the FHA portion but though it might help re: if you do decide to put more down and lease back. As @Brent Coombs outlined, COC of 21% isn't too bad either.