Thanks @MaryK4 .
So the steps are:
1. On page 1 of form 203, report all income (both NY and non NY) in fed column, report NY income in NY column.
2. Carry over total fed income to page 2, and calculate tax using total fed income.
3. Multiply the tax in step 2 by the ratio of NY income over total fed income, to get final NY tax.
Are those correct?
If form 203 carries over NY income instead of fed income to page 2, and calculate directly NY tax (without the ratio step), the tax would be actually lower. And this would be truly "NY taxes nonresidents only on tax from NY sources".
But I was told this (step1 to 3) is how NY tax works anyway.
Thanks for your help.
