Such association shall have power to determine the premium rates for each occupation or risk insured by it and to prescribe rates of cash premiums sufficient to cover losses incurred and current cost. The premium rate on each policy shall prevail for a full year but annually may be changed by the directors. The current cost herein specified shall be such an amount as is sufficient to cover the losses and expenses incurred and a premium reserve equal to 50 percent of all premiums and assessments levied or paid.