CSA 1099R Unknown - SURVIVOR BENEFITS

Box 2a of my CSA 1099R reads unknown, but I do not have any of the information necessary to use the simplified (or general) method to determine my taxable amount. (I receive these benefits from my former spouse.) Do i leave that field blank and proceed with my taxes? If not, how would I calculate the taxable amount when I'm not the one who the pension plan belongs to?

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