This question is pretty specific, but it's worth a try. Apologies in advance for the length; I'm aiming to be as clear as possible.
In 2020, as a provision of the CARES Act, it is possible to take the standard deduction on federal income tax and additionally deduct up to $300 worth of charitable contributions (Form 1040, Line 10). This $300 deduction is above the line, i.e., it reduces adjusted gross income (Form 1040, Line 11).
Enter California state taxes. On a California tax return, adjusted gross income (from Line 11 of the 1040) is the starting point, then you attach Schedule CA (540) (instructions) to make further adjustments to both income and deductions (for example, as required by discrepancies between federal and state law).
It's possible to itemize on California taxes while taking the standard deduction on federal taxes. This is done by checking the box at the top of Part II of Schedule CA (540) and attaching a federal Schedule A.
Here's where I start to get confused. The federal Schedule A asks on Line 11 for my total charitable contributions. Let's call that amount $X. Of course, if I were itemizing on my federal taxes, I'd only be able to claim the $X (on Schedule A) and not the $300 (on Form 1040). But because this Schedule A is just hypothetical -- it's not part of my actual federal tax return -- the $300 deduction is part of the adjusted gross income California is telling me to use, and then the attached Schedule A claims $X, which would result in $300 worth of double-dipping.
Now, form CA (540) should be where this gets ironed out. But the instructions are hard to follow (the relevant parts can be found by searching for "CCSD", which comes up in three places). In some places I literally don't know what the instructions are telling me to do. What especially baffles me is that the instructions for Part II (Adjustments to Federal Itemized Deductions), Line 11 seem to be telling me to write something in the Additions column (rather than Subtractions), which would seem to result in additional double-dipping (the "Additions" in Part II increase my deductions, not my income).
Can anyone walk me through what to do here and why it comes out right in the end?