A portion of my income had social security and medicare taxes withheld but not federal income taxes. In order to determine if I need to make an estimated payment to avoid underpayment penalties I am trying to figure out how exactly the 90% safe harbor number is calculated - i.e., do I need to pay 90% of the federal income tax I will owe or is it 90% of the total federal tax I will owe (including social security and medicare)?
