Tax code change after pension contribution

I paid in additional 3600 into my pension in 2022-23 and so 900 was credited to my pension and got 900 as SA tax relief (40% tax bracket). Subsequently, my tax code was increased by 450 for 2023-24. what calculation has HMRC used to come up with this figure of 450?

Hi,

We would need to access your record to review the tax code. You can contact HMRC at:

Income Tax: general enquiries

Thank you.

is there no general calculation that HMRC follows to change the tax code after additional pension contributions have been made in the previous tax year? Is it 50% of the tax relief or something else? Tried calling HMRC but it cut off saying that we are very busy