The $300 cash charitable deduction for those taking the standard duction is supposed to, indeed, show up on the Social Security taxable benefit worksheet, line 6. Turbotax says it is getting that line 6 number from Schedule 1, lines blah-blah-blah. Turbotax is actually getting that number from line 10b, form 1040-SR, and that is correct. If you have a 1040 instruction booklet (the old fashioned, printed copy) for 2020, it spells out that the SS worksheet, line 6, includes your $300 cash contribution. TT's source for line 6 simply does not show where the number is coming from.
