This is how I answered in the other thread:
Suppose you received $12,000 in SS benefits, and that because of other income, 85% of your benefits were taxed ($10,200).
When the federal numbers come over to CA, CA starts with the federal Adjusted Gross Income (AGI) on line 13 of form CA 540. In that AGI number, the $1,800 that is not taxed on the federal is not present - so the $1,800 is not taxed in CA.
And as Isabella pointed out, on line 14, the $10,200 will appear as a subtraction, to subtract out the taxable portion of the SS benefits on the federal return.
So the $1,800 is not taxed in CA and the $10,200 is not taxed in CA, and together these add up to the original $12,000. California does not tax SS benefits.
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