I can't site you the IRS regulation, but it is my understanding that the state tax deduction is limited to $10,000 for the NIIT. That is also the position of TurboTax. The form instructions provided by the IRS are only a summary of the full regulations, which are often subject to interpretation.
You do have the option to override the entry in the desktop version of TurboTax if your research indicates that you can deduct the full amount of state taxes. However, that would invalidate your Turbotax's accuracy guarantee.
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