For purposes of this subchapter, the notional shipping income of an electing corporation shall be the sum of the amounts determined under subsection (b) for each qualifying vessel operated by such electing corporation.
In the case of a qualifying vessel any of the income from which is not included in gross income by reason of section 883 or otherwise, the amount of notional shipping income from such vessel for the taxable year shall be the amount which bears the same ratio to such shipping income (determined without regard to this paragraph) as the gross income from the operation of such vessel in the United States foreign trade bears to the sum of such gross income and the income so excluded.
If for any period 2 or more persons are operators of a qualifying vessel, the notional shipping income from the operation of such vessel for such period shall be allocated among such persons on the basis of their respective ownership, charter, and operating agreement interests in such vessel or on such other basis as the Secretary may prescribe by regulations.