https://www.irs.gov/instructions/i8960 says:
Line 9b—State, Local, and Foreign Income Tax
Include state, local, and foreign income taxes you paid for the tax year that are attributable to net investment income. Form 1040-NR filers include only taxes paid for the U.S. residency period of the tax year. Sales taxes aren’t deductible in computing net investment income. You may not take a deduction for any foreign income taxes paid for the tax year if you took a credit for any portion of them. See section 275(a)(4).
You can determine the portion of your state, local, and foreign income taxes allocable to net investment income using any reasonable method. See Reasonable method allocations, earlier, and Deductions subject to AGI limitations under section 67 or section 68, later.
