The Federal Real Estate Settlement Procedures Act (RESPA) requires lenders to disclose APR, or annual percentage rate. There is only one acceptable method for Federal compliance. If your mortgage interest rate is at 5.75%, and your payment is a given amount, then the total amount of your mortgage, less any additional fees you are paying to the lender such as points, are deducted from your loan amount and the lower amount that results is used to back-calculate what the interest rate would be if your given payment were used to pay back the smaller amount. That rate is your APR. Theoretically this allows you to compare different combinations of rates and points, but in practice it isn't so useful.
An example: Your loan amount is $100,000 at 5.75%. That would make your payment for 30 years 583.57 per month. If you paid two points to the lender, then using $98,000 and a payment of 583.57 your APR would be 5.937%.