(b) Definition.--The city fiduciary adjustment shall be the net amount of the modifications described in section twelve (including subdivision (d) if the estate or trust is a beneficiary of another estate or trust), and in subdivision (c) and paragraphs two and three of subdivision (d) of section fifteen, which relate to items of income, gain, loss or deduction of an estate or trust. The net amount of such modifications shall not include any modification described in paragraph nine of subsection (c) of section twelve with respect to gains from the sale or other disposition of property, to the extent such gains are excluded from federal distributable net income of the estate or trust.

(c) Shares of city fiduciary adjustments.--(1) The respective shares of an estate or trust and its beneficiaries (including, solely for the purpose of this allocation, nonresident beneficiaries) in the city fiduciary adjustment shall be in proportion to their respective shares of federal distributable net income of the estate or trust.

(2) If the estate or trust has no federal distributable net income for the taxable years, the share of each beneficiary in the city fiduciary adjustment shall be in proportion to his share of the estate or trust income for such year, under local law or the governing instrument, which is required to be distributed currently and any other amounts of such income distributed in such year. Any balance of the city fiduciary adjustment shall be allocated to the estate or trust.

(d) Alternate attribution of modifications.-- The administrator may by regulation establish such other method or methods of determining to whom the items comprising the fiduciary adjustment shall be attributed, as may be appropriate and equitable. Such method may be used by the fiduciary in his discretion whenever the allocation of the fiduciary adjustment pursuant to subdivision (c) would result in an inequity which is substantial both in amount and in relation to the amount of the fiduciary adjustment.