In my personal budget, I allocate a fixed amount £x per month to mortgage repayments. This amount includes a sum which is going towards overpaying the mortgage so that I can get the thing over with as soon as possible. Each of these overpayments is above the threshold for reducing the subsequent monthly regular payment amount, so every month a larger proportion of £x is being allocated to overpaying and a smaller to the regular payment.
All mortgage overpayment calculators I've found assume you are overpaying a fixed £ or % amount each period and don't let you fix the total overpayment + regular payment amount, so it's hard for me to see how this strategy is going to play out. I'm interested in seeing how the payments are going to fall month on month, and how long it's going to take me in total to pay the thing off.
I played around with creating a spreadsheet using the IPMT family of functions but the results I came up with were always a few % off the lender's calculations even over one month, so over the remaining 23 years the figures wouldn't be very accurate.
If such a calculator exists I'd be more than happy to use it, or to receive advice on how to make a spreadsheet which has a good level of accuracy.