This is the Net Investment Income Tax to finance Obamacare.
Line 9b provides that I can deduct a large part of my state income tax (the part attributable to investment income, which is most of my 2020 income) from the amount subject to tax.
Include state, local, and foreign income taxes you paid for the tax year that are attributable to net investment income.
The consumer-level tax prep program I am using, which will remain unnamed, has filled in a small number based, as far as I can tell, on the $10,000 maximum state income tax deduction. Alternatively it may be based on my 2020 paycheck withholding. Both possibilities look wrong to me. Shouldn't it be based on the entire state income tax I will owe (including the estimated tax payment I made in January 2021)?