In addition to the deductions from taxable income otherwise allowed such a corporation, a China Trade Act corporation is, under certain conditions, allowed an additional deduction in computing taxable income. This special deduction is an amount equal to the proportion of the taxable income derived from sources within Formosa and Hong Kong (determined without regard to this section and determined in a manner similar to that provided in part I (section 861 and following), subchapter N, chapter 1 of the Code, and the regulations thereunder) which the par value of the shares of stock of the corporation, owned on the last day of the taxable year by (a) persons resident in Formosa, Hong Kong, the United States, or possessions of the United States, and (b) individual citizens of the United States wherever resident, bears to the par value of the whole number of shares of stock of the corporation outstanding on that date. The decrease, by reason of such deduction, in the tax imposed by section 11 must not, however, exceed the amount of the special dividend referred to in section 941 (b), and is not allowable unless the special dividend has been certified to the Commissioner by the Secretary of Commerce.