This is what you actually pay out of pocket each month once your tenants' rent is applied — compare it to what you'd pay renting an apartment on your own.
FHA applies a "self-sufficiency test" on 3-4 unit properties: 75% of the market rent for every unit (including yours) must cover the full monthly payment. This does not apply to duplexes. Actual underwriting varies by lender — confirm specifics with your loan officer.