TurboTax is calculating line 17b of the Maryland 502 correctly.
Line 17b shows the amount of state income tax that was included in your federal itemized deduction that needs to be added back because state income taxes are not deductible for Maryland tax purposes.
$0 on line 17b means that you don't need to reduce your state itemized deduction. This is because, in your case, your federal itemized deduction is made up entirely of your real estate tax since it's over $10,000. Maryland follows federal law when it comes to the limit on state tax deduction - you can only deduct up to $10,000 in state taxes. This is why the negative $20 was disregarded.
