The finance charge interest rate is APR (annual percentage rate), not MPR. So, you’ll divide the percentage by twelve before multiplying it with the total. If you're finance charge interest rate is 13%, you'll divide that by 12 (1.08333%) then multiply it by your total.
If you're total is $25.76, the monthly finance charge would be (25.76 * 1.08333%) $2.14
You can also set a Finance Charges Invoice threshold so that you're not generating invoices for minuscule dollar amounts. For example, you can set the threshold for $10.00, then you'll never generate a finance change invoice if the finance charge amount is less than $10.00. To set this, go to Settings > Financial