Some years ago my spouse consolidated his 401k plans and rolled them over to a Fidelity Rollover IRA. Last year he did his first Roth conversion of a portion of it. I forgot that for his first employer after-tax contributions were included in his 401k. And so left Line 17 of Form 8606 (basis on the amount converted) blank, when as I understand it now, it should have been a pro-rata amount (i.e, after tax amt/IRA current value x conversion amount), which is then subtracted from the Roth conversion to obtain the taxable amount of the Roth conversion.
Should I enter a amended return for 2019 or can the overpayment of taxes on the first Roth conversion simply be used to adjust the basis of the remaining Rollover IRA? If I need to do the first alternative (amended return for 2019), do I need to do that before filing 2020's return which will have a Roth conversion?
