(a) A resident taxpayer shall be allowed a credit against the tax otherwise due under this act 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 by another state with respect to income which is also subject to tax under this act.
(b) The credit provided under this section shall not exceed the proportion of the tax otherwise due under this act that the amount of the taxpayer's taxable income subject to tax by the other jurisdiction bears to his entire taxable income.