Net earned premiums on marine insurances written within the United States during the calendar year shall be computed as follows:
a. Gross premiums on marine insurances written within the United States during the calendar year, less any and all return premiums, any and all premiums on policies not taken and any and all premiums paid for reinsurance;
b. Add unearned premiums on such outstanding marine business at the end of the preceding calendar year; and,
c. Deduct unearned premiums on such outstanding marine business at the end of the current calendar year.