What is the formula for interest if payments change?

I am looking for a formula for interest to use in excel to come up with a new amortization schedule for my mortgage. I recently made a large payment to reduce the principle on my mortgage, however I am still going to continue to make the same payment I did last year to further reduce my principle each month. Is there a formula for interest other than the traditional one based on Principle, period, and interest rate?

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