Hi Richard,
Problem with datawindow is: You can access values in columns from previous row but not from computed fields. But loan amortization schedule calculation needs to access calculated values from previous row.
If you don't want to do coding outside the datawindow you can only use one single row with a very large detail band for the whole schedule. You need computed fields for every payment period in this single row. This is very inflexible because you have to define a maximum number of periods at design time. And you will get problems to print the schedule.
So the best way is to do the calculation outside the datawindow and use datawindow only to show the result.
HTH,
René