Calculation Formula Reuired

Lee-10

Good Evening 

Apologies for the message, I am currently 1 year into a 5 year fixed rate of 4.57% current balance £225,000.

I have been making small regular overpayments. However I want to shift focus to get my mortgage payment lower (short term goal). I understand repaying capital is better

So I spoke to Nationwide and they said for overpayments over £1000 it will reduce the following months payment (hope that makes sense)

Does anyone know the formula so I can work out how much my payments will reduce by? and what I need to pay off to get my payments under £1000

Thanks for your Help in advance 

Lee