The total of the state, local, and foreign income taxes that you paid for the current tax year is entered on line 9b of form 8960. Enter only the tax amount that is attributed to the net investment income. Do not include sales tax or any foreign income taxes paid for which you took a credit.
How do I determine the amount that is related to my investment income?
You can determine the portion of your state, local, and foreign income taxes that is allocated to the investment income using any reasonable method. Examples of reasonable methods of allocation include, but aren’t limited to, an allocation of the deduction based on the ratio of the amount of a taxpayer's gross investment income to the amount of the taxpayer's AGI.