(See my other comment)'
You mention - "the amount on my [MAGI] worksheet does not include any social security retirement income, taxable or non-taxable, which is odd. " I have same question.
TT did calculate my MAGI for 'passive gain/loss' calculation (on the MAGI Worksheet) but like you, it did not include any SS income (neither taxed or untaxed portions). I don't know if TT did this correctly? (It wouldn't be the first error I've found in TT) Assuming its correct for passive gain/loss purposes, I also need MAGI for Medicare Part B IRMMA rate planning, and need to know if I should add SS income to the MAGI for IRMMA purposes or not? I find conflicting references, and the IRS forms are only vague in addressing this matter.
