Each civilian member who retires under section 8584 of this title is entitled to a life annuity computed by multiplying his average annual compensation during any five consecutive years of allowable service, at his option, by his number of years of service, not exceeding 35, and dividing the product by 70. The retirement annuity payable to a retired civilian member under a policy required by section 8582 of this title is counted as part of the retirement annuity provided in this section. Any difference between the amount received by the retired civilian member under his annuity policy and the total annual amount to which he is entitled under this section shall be paid to him by the Secretary of the Navy from appropriations made for this purpose.