Self assessment advice - underpaid tax estimate or actual??

Hasher

Hi. Hoping someone can help with this dilema.

Someone has, in April 2026, been sent a letter from HMRC requiring them to register for self assessment for the first time and provide a return for 24-25.
They are employed, with no other income except bank interest that exceeds 10k.

The self assessment form says this:
Underpaid tax included in PAYE coding (Page 2 of 2)
Enter the amount of estimated underpaid tax for 6 April 2024 to 5 April 2025 to be included in your PAYE tax code for 6 April 2025 to 5 April 2026 (from your PAYE Notice of Coding). Enter '0' if you do not have underpaid tax for the tax year:


If your 6 April 2024 to 5 April 2025 tax code was reduced during the year because of a change in circumstances, you may not have paid enough tax.

Look at your 6 April 2024 to 5 April 2025 PAYE Coding Notice (P2).

If it includes the message 'our estimate of the underpayment for the earlier part of the year is £xxxxx' then put this amount in the box above.

Question is, do they include that estimated value that was in the 25-26 tax coding, or, because we know the actual amount of tax due for 24-25 do we put that figure.

To illustrate.
Tax coding dated 12 March 25, says in note 4 'we previously told you you owe us £1249, we have included an adjustment to to reduce your tax free allowance by 6246 to collect 1249.

But having calculated the tax owed for 24-25 it comes to £945.

So not sure which figure to include, 1249 or 945.
Grateful for any advice.