I know that 15% of my social security income is not taxed at the Federal level. But is that 15% a deduction or an adjustment? That is, is the 15% counted in AGI or not? I get gobbledegook when I google this question.
The amount of social security income not subject to Federal income tax (at least 15%, but varying based on total base income) is not included in your AGI in normal annual filings.
On the current form 1040, total social security benefits are entered at the left on line 5a, while the taxable portion (85% or less) is entered at right in 5b per the worksheet on page 33 of the instructions. AGI is the sum of taxable income in the right hand column; lines 1, 2b, 3b, 4b, 5b, and adjustments from schedule 1.