For my own mortgage I only use that method to give a simple reckoning of the effect of various interest rates if I pay the required payments.
The proper model has actual and projected payments plotted per month with interest, then I chart outstanding balance against calendar date. Then I can override future payments (or rates) and see when balance hits zero, how curve changes etc.
Yep but the original data has constant payments.
For my own mortgage I only use that method to give a simple reckoning of the effect of various interest rates if I pay the required payments.
The proper model has actual and projected payments plotted per month with interest, then I chart outstanding balance against calendar date. Then I can override future payments (or rates) and see when balance hits zero, how curve changes etc.