Haw. Rev. Stat. § 237-21: Apportionment.

If any person, other than persons liable to the tax on manufacturers as provided by section 237-13(1), is engaged in business both within and without the State or in selling goods for delivery outside the State, and if under the Constitution or laws of the United States or section 237-29.5 the entire gross income of such person cannot be included in the measure of this tax, there shall be apportioned to the State and included in the measure of the tax that portion of the gross income which is derived from activities within the State, to the extent that the apportionment is required by the Constitution or laws of the United States or section 237-29.5. In the case of a tax upon the production of property in the State the apportionment shall be determined as in the case of the tax on manufacturers. In other cases, if and to the extent that the apportionment cannot be accurately made by separate accounting methods, there shall be apportioned to the State and included in the measure of this tax that proportion of the total gross income, so requiring apportionment, which the cost of doing business within the State, applicable to the gross income, bears to the cost of doing business both within and without the State, applicable to the gross income.