When you indicate that the contribution was made in 2024 then it will be deducted from your useable basis in 2023. You can see this on Form 8606 lines 4 and 5. Therefore, it will affect the calculation for the nontaxable part of the conversion you made in 2023 (reported on the 2023 Form 1099-R) and then affect the amount of tax due. If you indicate that you made the nondeductible contribution in 2023 then your nontaxable portion will be bigger and therefore the taxes due are reduced.
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