Start with the balance record
Record the original invoice amount and each payment, credit or correction that changed the balance. Keep the source for every change.
Record the date inputs
Use the due date shown by your underlying record and the calculation date you selected. If the due date is uncertain or disputed, flag that issue before relying on the result.
Treat the rate as an assumption
Record the rate entered, why it was selected and who checked it. Do not present a calculator default as a conclusion that the rate applies.
- Principal balance used
- Recorded due date
- Calculation date
- Rate entered and its source
- Payments or credits reflected
Keep the result with its inputs
Save or copy the result together with the assumptions and calculation date. If an input changes, produce a new calculation record rather than overwriting the earlier one.