Box 9b cannot normally be used in calculating the taxable amount because it indicates that total amount of after-tax contributions to the plan, not just the portion of after-tax contributions included in the distribution.
If box 2a has an amount in box 2a and box 2b Taxable amount not determined is not marked, the amount inbox 2a is most likely the correct taxable amount and the IRS will question any amount that you indicate as the taxable amount that is lower than the amount in box 2a. The amount in box 2a is generally the amount in box 1 minus any amounts in boxes 5 and 6.
