Deductions & credits

Yes. It is still correct. New York uses your total income from everywhere (after NY adjustments) to come up with NY adjusted gross income.

Since all your income was included in the calculation of NY AGI, your total NY itemized deductions are subtracted to arrive at NY taxable income.

NY then computes a base tax, which is the tax you would pay if all your income was earned in NY. Your actual tax is the base tax multiplied by your percentage of NY income.

For example, if you earned $100,000 everywhere, then your NY base income would be $100,000. If the NY tax on $100,000 is $x and you earned $50,000 in NY then your part-year/non-resident tax would be 50% ($50k/$100k) of $x.