Editing this post after completing the Form 8959 manually to gain a better understanding.
Thank you, this uncovers the source of the number $412, it is line22 of Form 8959. In my case (filing married jointly) it is effectively what was withheld from W2 earnings (box6 of W2) above the base Medicare tax rate of 1.35% applied to Medicare wages and tips (box5 of W2). Part I calculates the additional medicare tax owed.
It's a little bit of funny math, but the way TurboTax displays it makes sense if you understand the underlying tax forms.
From my point of view, when I first approached trying to verify the number, I took box5 from the W2 and the Medicare tax rates to calculate my total Medicare tax bill. Then I subtracted what had been withheld from my pay checks throughout the year (box6 from W2), and the difference should be when I owed to Medicare tax.
I think this would be a better way to display the remaining tax owed for Medicare in the "Explain my taxes" section. Or, if left as is, give some more context to what "Other withholdings" and "Additional Medicare tax" mean, because it is very misleading to someone who doesn't have a firm understanding of Form8959.
