Reusable Excel Payment Calculator

I need a small, tidy Excel model that lets me drop in a few client-specific numbers, instantly shows the payments that fall due over time, and can then be copied into any other workbook without breaking.

The core of the task is to translate the custom mathematical formula I’ll share (it’s straightforward once you see it) into dynamic Excel logic. Most of our agreements are monthly, but the file has to stay flexible enough to handle other periods whenever an arrangement differs. When I paste the block into a fresh sheet for a new client, everything should still work: all links remain internal, no hard-coded references to external files, and totals recalculate automatically.

Key points you’d be tackling:
• Build the calculation engine exactly as provided, using standard Excel functions only—no macros or VBA.
• Make the period selector adaptable so I can toggle between month-by-month schedules or a custom timeline.
• Keep inputs, outputs, and helper cells clearly separated so anyone can follow the flow.
• Finish with a short “How to copy” note right inside the file to guide our team.

Hand back a clean .xlsx with one worked example; once I confirm it matches the formula I supply, we’re done.