I think I understand what you ask. You just need to account for the difference but it is a moving target each month (assuming you plan on overpaying each month).
Below example is just for simple illustration (
figure does not reflect reality, just nice round figure for ease of understanding)
Example:
- Current payment is 1,000, lets say you overpay by 500 (and you plan to keep overpay the same amount) and new payment is 990.
- For next payment, the bank will take 990 so you need to make overpayment by 510 and your new payment is 980.
- For next payment, the bank will take 980 so you need to make overpayment by 520 and your new payment is 970.
- The cycle repeats....
The easiest way is if you know the specific total payment you will consistently want to make, as long as your payment add up to that amount, that's really all that matters. You can work it out because every time you overpay, the bank will send you a letter telling you what the new figure is that they will take next month.
Do i have to put my original monthly amount put back every time i over pay to get the benefit of overpaying
Yes and no. It depends on what you are trying to achieve here.
Yes: If you overpay once and leave the bank to take the new repayment value, you will save on interest of the overpayment amount you made over the remaining lifetime of the mortgage.
No: To take the full benefit, you should at least try to keep the monthly repayment total (i.e. new repayment figure + overpay to get back to the original figure) the same as it was before, in my example above, that would be 1,000 (This would be near equivalent to reducing the "term" at the end of the mortgage. While the process is a little bit of manual work to keep an eye on the total amount each month but you get the benefit of reducing the "term" but maintaining the flexibility of stopping the overpay portion should you get into any financial difficulty). This is why it is generally recommended to not reduce the term with the bank but rather overpay because the bank will not extend the term when you need it later on.