How does TT get the amount of state & local income taxes paid on line 5a of Sched A? Much lower than 2016 & 2017 actuals. Can I use the correct $? Which year do I use?

State & local income taxes paid on your 2017 tax return are for state & local income taxes paid in 2017. To see where these payments are coming from, and to enter or change these payments to the correct number if needed see: https://ttlc.intuit.com/replies/3302371

But where does that number come from? None of the items listed on that page above applt to me.  I just have a w2 but the number from that does not equal what TT entered in line 5a.

*apply

you can deduct whatever you have receipts for. That's the IRS rule. keep with your other tax documents.

oops. misread your question. amend, if your sched A is off by a significant amount. it has to be worth the hassle.