Based on what you are saying
1) I should not be paying an underpayment fee.
If the Turbotax calculation is correct, I've paid 98% of my owed nanny taxes
2) Turbotax then is clearly is assessing my penalty based on what it thinks I have not paid on my nanny taxes, and is not connecting that the estimated taxes I paid are for the nanny tax.
Amending my W4 would do nothing to solve this now or in the future, as that's based on adjusting my personal income taxes withheld, not what I am withholding from my nanny as the employer or what taxes I've already paid as the employer of my nanny.
I am still in the same boat... Turbotax is having me pay a an underpayment fee for something I've already paid.
