he acts in the capacity of a general chairman or an assistant general chairman of a general committee which represents individuals rendering service in the United States to an employer, but in such case if his office or headquarters is not located in the United States and the individuals represented by such general committee are employees of an employer not conducting the principal part of its business in the United States, only such proportion of the remuneration for such service shall be regarded as compensation as the proportion which the mileage in the United States under the jurisdiction of such general committee bears to the total mileage under its jurisdiction, unless such mileage formula is inapplicable, in which case such other formula as the Railroad Retirement Board may have prescribed pursuant to section 1(c) of the Railroad Retirement Act of 1937 ( 45 U.S.C. 228a ) shall be applicable, and if the application of such mileage formula, or such other formula as the Board may prescribe, would result in the compensation of the individual being less than 10 percent of his remuneration for such service, no part of such remuneration shall be regarded as compensation;