Not my spreadsheet but this is a useful one for comparing overpayment, deals, etc http://www.locostfireblade.co.uk/spreadsheet/Index.html
But, you do need to compare it with what the overpayment would be earning if you did something else with it.
@aggi started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
Not my spreadsheet but this is a useful one for comparing overpayment, deals, etc http://www.locostfireblade.co.uk/spreadsheet/Index.html
But, you do need to compare it with what the overpayment would be earning if you did something else with it.