google "The Pro-Rata Rule for Backdoor Roth IRA’s

• you may owe tax on a nondeductible Traditional IRA to Roth conversion if you have money remaining in other “pre-tax” IRA accounts.

If on 12/31/2021 you have such pre-tax IRA money, your tax amount for your backdoor roth convertion should use Pro-Rata rule to calculate and you'd fill form 8086.

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