Additional Medicare Tax, Witholding, Safe Harbor

So we have unpredictable earnings, though most of it is realized in the early part of the year when bonus is paid.
I try to have 110% of last year's taxes withheld to meet the safe harbor requirements, though sometimes an estimated payment is due in Q4.
Last year when the Additional Medicare Tax (0.9%) was new, I didn't realize that it counted as part of the withholding until I used HR Block Tax Cut to estimate my penalty if I didn't make a Q4 payment, and found that I didn't have any.
Now I'm trying to calculate 1) What my safe harbor is, and 2) What counts as withholding.
My understanding is that my safe harbor is 110% of Form 1040, Line 61 (Total Tax). This included both the Additional Medicare Tax on wages (Form 8959), and the Net Investment Income Tax (Form 8960).
Is that correct?
Now my paycheck doesn't separate out the Medicare Tax (1.45%) from the Additional Medicare Tax (0.9%), but I can certainly calculate how much Additional Medicare Tax is withheld.
In calculating the total amount withheld, I believe I can include this Additional Medicare Tax (0.9%) in the total.
Is that correct?
As I try to forecast my total income, and adjust my withholding to meet the safe harbor, I just want to make sure I'm doing it correctly.
Thanks in advance.
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snipped-for-privacy@yahoo.com wrote:

Yes.

Yes. The additional medicare tax is just normal federal tax that showws up on line 2 of the W-2. To pay more of it just decrease the number of exemptions on the W-4 or if you are at zero exemptions then have additional tax withheld.
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On 2014-05-05 20:58, remove ps wrote: [...]

I agree with the "Yes" answer, but the rest of the reply is a little fuzzy to me.
Add'l Medicare Tax from Box 6 of the W-2 is included in Form 1040, line 62 as part of your total federal withholding, which is what you need to know when calculating your "safe harbor" for underpayment of estimated tax interest (a.k.a. penalty) charges. How you increase or decrease your other withholding or estimated tax payments is up to you, but this particular withholding (0.9% Add'l Medicare tax in Box 6) you have no control over.
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Mark Bole wrote:

All I'm saying is that the additional medicare tax is just a federal tax and to pay it you have to either increase your additional federal withheld and/or decrease your exemptions. I'm not clear how the original poster separated the medicare tax (I suppose on line 6 of the W-2) into the base portion and the 0.9% portion. That line 6 is always the base portion.
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On 2014-05-07 08:20, remove ps wrote:

The reconciliation portion of Form 8959 shows how it works. W-2 Box 6 includes all Medicare withholding, not just the "base" portion.
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Mark Bole, EA
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