The NJ credit is not a percentage of the tax paid to the other jurisdiction. It's the amount of NJ tax that you would have paid on the income if it had been NJ income instead of PA income. You can see the calculation of the credit on Schedule NJ-COJ in your New Jersey resident tax return.
The credit and the reciprocal agreement are two separate things. You use one or the other, not both. Which one applies to you depends on whether your PA income is from a job where you get a W-2, or from any other source. If you are a W-2 employee you should not have paid any Pennsylvania state income tax. Under the reciprocal agreement between NJ and PA you pay tax only to the state that you live in. If PA tax was withheld from your pay you have to file a Pennsylvania nonresident tax return to get a refund of the tax that was incorrectly withheld. You should also fill out Form REV-419 and give it to your employer so that they stop withholding Pennsylvania income tax.
If you had self-employment income from PA, or any other PA income besides a W-2, then you would file a PA nonresident tax return to pay PA tax on the income, and you can claim the credit for tax paid to another jurisdiction on your NJ resident tax return. For more details about the NJ credit see New Jersey Bulletin GIT-3W. The following is a short excerpt from that bulletin.
"This credit minimizes double taxation of income that is already taxed by other jurisdictions. Though the credit reduces your New Jersey Income Tax, you are not necessarily entitled to a dollar-for-dollar credit for the total amount of taxes paid to the other jurisdiction. The credit represents the amount of New Jersey Income Tax that you would have paid had the income been earned in New Jersey, or the amount of tax actually paid to the other jurisdiction, whichever is less."
