A resident beneficiary of an estate or trust who is taxable on the income of the estate or trust under Chapter 9 of this part shall, subject to the following conditions, be allowed a credit against the taxes imposed by this part on such income for net income taxes paid by the estate or trust to another state on such income:
(a)
Credit shall be allowed only for such proportion of the tax paid to the other state by the estate or trust as the income of the estate or trust which is taxable to the beneficiary under this part and also taxed to the estate or trust in the other state bears to the entire income of the
estate or trust upon which the taxes paid to the other state were imposed.
(b)
The credit shall not exceed such proportion of the tax payable under this part as the income of the estate or trust which is taxable to the beneficiary under this part and also taxed to the estate or trust in the other state bears to the beneficiary’s entire income upon which the tax is imposed by this part.