The amount determined under subsection (a) shall be treated as consisting of the same proportion of each class of items entering into the computation of distributable net income of the estate or trust as the total of each class bears to the total distributable net income of the estate or trust in the absence of the allocation of different classes of income under the specific terms of the governing instrument. In the application of the preceding sentence, the items of deduction entering into the computation of distributable net income (including the deduction allowed under section
642
(c)) shall be allocated among the items of distributable net income in accordance with regulations prescribed by the Secretary.