Method of taxing residents.

If a person is a Commonwealth resident, income from sources both within and without this Commonwealth shall be subject to the Com monwealth personal income tax. A taxpayer shall be allowed a credit against the personal income tax if subject to and paying any tax imposed by other states or countries which is based upon income and such income is subject to the Commonwealth personal income tax. This credit shall be limited to the proportion of the tax otherwise due that the amount of the income of the taxpayer subject to tax by the other jurisdiction bears to his entire taxable income. If a taxpayer claims a credit for taxes paid to other states, references should be made to the detailed instructions of Schedule G.