we're saying the same thing for that question - you report your balance as of 12/31, do not adjust for any contribution made for 2024 after 12/31.
This 12/31/24 balance is used to calculate the tax on any Roth conversion you did in calendar year 2024; if you have a balance (across all your IRAs if your multiple accounts) in excess of the basis from the "after tax" non-deductible contributions (due to earnings in the IRA or a 401k rollover etc), then the Roth conversion will consist of a pro-rata amount of pre-tax and after-tax funds, and taxed accordingly. The general premise of the backdoor Roth being entirely non-taxable is you do not have any balance in your Trad IRA, you contribute, convert and it goes back to zero.
Separately, you would report that you made a contribution in 2025 for 2024, which will update your basis in the IRA for the 2024 tax year which will carry forward to the tax calculation in 2025. This will be reflected in lines 1, 4 and 14 of Form 8606 for 2024, and carries forward to line 2 of Form 8606 for 2025.
If you are using desktop use forms mode to see how Form 8606 is being filled in; with TT online I think you can see it when you export the PDF of your return.
