"Federal reg. allows a deduction of $10K state and local tax on schedule A."

Are you asking about the Limited deduction? In other words, you enter the taxpayer Actuals and if that exceeds the limit, they are not allowed all of it. Otherwise, they are Allowed the amount they incurred and entered.

See this: https://www.irs.gov/taxtopics/tc503

And: https://www.irs.gov/taxtopics/tc501

And: https://www.irs.gov/help/ita/can-i-deduct-personal-taxes-that-i-pay-as-an-itemized-deduction-on-sche...