(a) A resident taxpayer before allowance of any credit under section 312¹ shall be allowed a credit against the tax otherwise due under this article for the amount of any income tax, wage tax or tax on or measured by gross or net earned or unearned income imposed on him or on a Pennsylvania S corporation in which he is a shareholder, to the extent of his pro rata share thereof determined in accordance with section 307.9,² by another state with respect to income which is also subject to tax under this article. For purposes of this subsection, the term “state” shall only include a state of the United States, the District of Columbia, the Commonwealth of Puerto Rico and any territory or possession of the United States.

(b) The credit provided under this section shall not exceed the proportion of the tax otherwise due under this article that the amount of the taxpayer's income subject to tax by the other jurisdiction bears to his entire taxable income.