Hello,
I would like to help you with your mortgage calculator.
After looking on the attached spreadsheet and your latest comments, I think that having a wizard for the user's input is a good idea.
All the formulas, used on the calculations, could be stored on a database table, Hence your online calculator will be dynamically driven.
Your online calculator could offer the feature of exporting the calculation results onto a CSV file which could be opened with Excel , by the end user.
Best Regards,
Marlon