With regard to determining your marginal tax rate, fanfare's post should generally be referring to checking the amount of income taxable as ordinary income rather than AGI. That could be the amount on Form 1040 line line 15, but there are also several other ways that the amount of income taxable as ordinary income is determined, such as on Schedule D Tax Worksheet or the Qualified Dividends and Capital Gain Tax Worksheet. AGI does matter if you are subject to Medicare IRMAA and need to consider the IRMAA thresholds, if you are approaching the $200,000 AGI threshold where Net Investment Income Tax kicks in, or if you receive any tax credits that depend on AGI.
