The "backdoor" provides the maximum benefit only if you have no other funds in traditional IRAs at the end of 2021. If you have other funds in traditional IRAs your Roth conversion would be largely taxable and some of your basis in nondeductible traditional IRA contributions will remain with your traditional IRAs to be applied to future distributions or Roth conversions. Your 2020 tax return will include Form 8606 to report only the nondeductible traditional IRA contribution. The Roth conversion will be taxable in on your 2021 tax return with the taxable amount calculated on the 2021 Form 8606. Line 14 of your 2020 Form 8606 will carry forward to line 2 of your 2021 Form 8606 to be used in that calculation.
