Each year the corporation shall set apart as earned surplus not less than 10 percent of its net earnings for the preceding fiscal year until the surplus equals one half of the amount paid in on the capital stock then outstanding. Whenever the amount of the surplus is less than the required amount, it shall be built up again to the required amount in the manner provided for its original accumulation. Net earnings and surplus shall be determined by the board of directors, after providing for reserves the directors consider desirable, and the determination of the directors made in good faith shall be conclusive on all persons.