The tax is not accurate historically, for higher income brackets. Prior to 2018(?) you could deduct the majority of your state income tax from the federal income tax (the "Trump tax cuts").

https://nypost.com/2021/10/05/court-upholds-trump-law-limiti...

Basically, prior to that implementation, you were really paying MAX(federal income tax, state income tax). After that implementation, you could only deduct a maximum of $10,000 in state income tax from federal tax. So it looks more like: ADD(federal income tax - $10,000, state income tax)