Box 9b is informational only and represents the amount of after-tax funds that remained in the plan prior to the distribution being reported on this Form 1099-R. If box 1 has $4,572 and box 2a has 4,439.28, the after-tax portion of your distribution was $132.72 and this number should be present in box 5.
Since there is an amount in box 2a, 2b Taxable amount not determined would not normally be marked and the payer is reporting the box-2a amount as the correct taxable amount. Only if you disagree that this is the correct taxable amount would you tell TurboTax that you need to calculate the taxable about or that another amount is taxable. With the correct amount in box 2a, you should not be using either the Simplified Method or the General Rule to calculate a different taxable amount.
