(a) Credit may be allowed only for taxes imposed by and paid to another state on income from personal services performed within such state, from land or other property located therein, from business carried on there, or otherwise derived from sources within such state and taxable under the laws of such state irrespective of the residence or domicile of the recipient. Thus, credit may not be allowed for taxes paid to another state on income from sources without such state even though such income may be taxable by such state, either because the taxpayer is domiciled therein or is considered a resident of such state notwithstanding that he is also considered a resident of this State. This limitation is not applicable to an estate or trust which, within the meaning of the law, is considered a resident of this State and also of another state. (See Sections 18003-04.)

(b) Credit may not be allowed for taxes paid to a state which allows nonresidents credit against the taxes imposed by such state for taxes paid or payable to the state of residence. In such case credit should be obtained from the state imposing a tax upon residents of this State.

(c) The credit shall not exceed that proportion of the tax payable under the law as the income taxable in such other state and also taxable under the law bears to the total income taxable under the law. In the application of this limitation only taxable income to which the taxpayer is entitled shall be considered.