To allow over or under-payment of debts, we include two fields which are the regular payment and the interest rate. The mortgage is increased each year with the interest rate and decreased by the amount of regular payment.
We annualise the payment based on the frequency entered and then apply it monthly to the debt. If the payment doesn't match the interest rate applied, there will either be a residual loan at the end, or it will pay off early, depending on if the payment is over or under the correct amount.