Use a break-even deadline
Start with how long you expect to keep the mortgage. Then ask for the highest refinance rate that still reaches break-even before that date. A lower rate gives more margin for fees or a shorter stay.
Use a savings target
Set a net financial-benefit goal, not only a payment reduction. The calculator compares payments made and remaining debt, so a longer term cannot quietly look better simply because its payment is lower.
Enter your numbers in the refinance calculator, then use Target savings or Break even by month under Advanced simulations.