If you just need the monthly payments, use the PMT function. To deal with
the balloon payment, NPER should be the number of months until the balloon
payment, and the FV will be the amount of that payment.
For example, if your initial loan is for $350,000, the balloon payment is
$300,000 due in five years (60 months) and the interest rate is 6% (.5% per
month), the monthly payment is calculated as =PMT(0.5%,60,-350000,300000).
