Splitting the plan under the QDRO, resulted in your ex receiving after-tax basis in the same proportion as the split of the overall pension. For example, the the pension was split 50/50, 50% of the after-tax basis went to your ex and 50% remained with you. Since box 9b shows that total after-tax basis that existed before the split, you would use the appropriate proportion of that amount with the Simplified Method as your investment in the plan. Your ex would do the same.
Using a 82/18 split of $200,000 of the basis, your portion of the basis would be $164,000 and your ex's portion of the basis would be $36,000. In TurboTax, "Plan Cost" is where your would put your portion of the after-tax basis .
If you received pension payments prior to 2023 and the split occurred after you began receiving the payments, you'll also need to proportionately adjust the Tax-Free Amount Previously Recovered so that it agrees with your 82%.
