Change one assumption at a time
To understand what drives a result, keep most inputs the same and change only one variable — such as rate, term, down payment, or extra payment. If several inputs change at once, it becomes difficult to tell which change created the difference.
For actual loan offers, compare APR, lender fees, discount points, mortgage insurance, and cash to close in addition to the calculator results. Those costs may not be fully represented by a simplified scenario.