After I posted my initial message, I noticed that the $300 entry on SS Benefits worksheet line 6 represents the charitable contribution I entered on 1040 line 10b. After consulting the IRS instructions for the worksheet, I learned this is appropriate. However, the TurboTax version of the worksheet mentions only amounts from Schedule 1 and fails to mention amounts from 1040 line 10b. That omission is what lead to my confusion.
