So you take the net rental income from the appraiserâ€™s estimate of fair market rent from all units, including the unit you will be occupying, less the appraiserâ€™s estimate for vacancies or 15% (whichever is greater). That amount can't be less than the monthly housing payment (principal, interest, taxes, insurance, mortgage insurance, and HOA fees, if any). If that amount is less than the monthly housing payment, then a down payment greater than 3.5% would be needed (to get that amount to no less than the housing payment).
FHA also requires 3 months PITI in reserves (cannot be gifted).
For 2-units, the "self-sufficiency" requirement & 3 months PITI in reserves are not applicable.
Qualifying income is different though, qualifying income from the rents can only be used if they are actually rented out, and of course the unit you will be occupying cannot have it's rental income help qualify either.
Shane Milne | Lending in all 50 states | NMLS #81195
The information I have been able to find supports the 85% rule of thumb you mentioned with 15% to cover for vacancies.
Let me know if I can be of further assistance in helping you in your buying search.