In determining the amount of the tax imposed by this chapter, net earned premiums on ocean marine insurance contracts written within the United States during the taxable year shall be arrived at by deducting from gross premiums written on such contracts during the taxable year all return premiums, premiums on policies not taken, premiums paid for reinsurance of such contracts and net unearned premiums on all such outstanding contracts at the end of the taxable year, and adding to such amount net unearned premiums on such outstanding marine insurance contracts at the end of the calendar year preceding the taxable year.