New York uses one's federal return as a starting point for determining state AGI and TI. But I have discovered many differences over the years and continue to find new ones from year to year, including some for 2020.
When I did my 2008 returns, the ones which included the company stock payout, I saw that NY didn't impose that 10% penalty for early withdrawal (before age 59.9) from retirement accounts. I already knew NY taxed all favorably taxed QD and LTCG as ordinary income.
I am doing my 90-yo dad's tax returns this time and have discovered some more differences. Nearly all of his income is not taxable, including SS (even if some of it ends up being federally taxable), his IRA withdrawals, and his small pension. I doubt he has paid any state income tax since he retired in 1994. In fact, he has been building up a LTC Premium tax credit which can be carried over from year to year if not taken in a given year.
For 2020, have any of you fellow New Yorkers notices this new form, IT-558, which adjusts one's already adjusted federal AGI to what they call a "Recomputed federal AGI?" Form IT-558 ignores and federal tax law changes stemming from the 2020 CARES Act such as disallowing cash/check charitable contributions up to $300 claimed by those who filed the standard deduction on their federal returns. So, I have to complete IT-558 to post the $105 I donated to charities, then add it back on the main form (IT-201) to get my "Recomputed federal AGI."