In some cases, an allowable deduction may relate to both apportionable income and to income that Maine is prohibited from taxing by the laws or Constitution of the United States, or by the Constitution of Maine. 36 M.R.S. §5200-A(2)(A) and (F). In such cases, the deduction must be prorated between apportionable income and exempt income in a manner that fairly distributes the deduction among the classes of income to which it is applicable.
18-125 C.M.R. ch. 801, § 11