8959 computes the additional medicare tax on wages over the threshold. this gets added to your tax liability
to your regular federal withholding, if your employer withheld more than 1.45% on your medicare wages that gets added to your federal withholding
say lines 1 and 4 of 8959 are $350,000
line 6 would be $100,000 for line 6 MFJ
and line 9 would be $900
$350,000 in medicare wages should normally have tax withheld @ 1.45% or $5,075 in this case if more than that was withheld that extra would be added to your federal withholding. part V of form 8959
that's the law. an extra .9% tax on medicare wages in excess of the threshold
in addition to an additional 3,8% tax for excess investment income.
