I always start with the current rent. Compare that with market. If it's vacant then I would see how many nearby are for rent. From there work backward with the expenses.
Linda — you're asking the right question, and the answer is: think like the underwriter, because the underwriter only cares about one ratio.
For DSCR, income is the LOWER of (a) your actual lease or (b) the appraiser's market-rent opinion. The underwriter takes that number and divides it by PITI — principal, interest, taxes, insurance. That's the DSCR. Most lenders want 1.0 or better, meaning the rent covers the payment.
So before you apply, run it yourself: market rent (be conservative — use the appraiser's likely number, not your best case), divided by the full PITI at today's rate. If it clears 1.0 with a cushion, it's worth pursuing. If it's under 1.0, some programs will still do it (no-ratio-required options exist) — but know which bucket you're in before you apply, because it changes the lender list and the pricing.
Operating expenses matter for YOUR analysis (vacancy, maintenance, capex decide whether the deal actually makes money), but they don't enter the DSCR math — the lender's formula is rent vs. PITI, full stop.
What rent and payment are you looking at? Happy to run the ratio with you — Dan