PA Thrift Savings Plan

I live in Pennsylvania and I am filing my state taxes.  I started withdrawing $2,500 each month starting in July.  I had $500 withheld for federal taxes.  It is my understanding that I do not have to pay taxess on whole amount withdrawn since I already paid  PA state taxes when contribution made to Thrift Savings Plan.  How do I use general rule to figure how much I pay tax on?

                                                     DaveD