In any election heretofore or hereafter held under the Act of June 18, 1934 (48 Stat. 984) [25 U.S.C. 461 et seq.], on the question of excluding a reservation from the application of the said Act or on the question of adopting a constitution and bylaws or amendments thereto or on the question of ratifying a charter, the vote of a majority of those actually voting shall be necessary and sufficient to effectuate such exclusion, adoption, or ratification, as the case may be: Provided, however, That in each instance the total vote cast shall not be less than 30 per centum of those entitled to vote.