For purposes of determining his taxable income for any taxable year, a taxpayer insured by a mutual fire or flood insurance company under a policy for which the premium deposit is the same regardless of the length of the term for which the policy is written, and who is entitled to have returned or credited to his on the cancellation or expiration of such policy the unabsorbed portion of the premium deposit not required for losses, expenses, or establishment of reserves, may, if such amount is otherwise deductible under this chapter, deduct so much of his premium deposit as was absorbed by the company during the taxpayer's taxable year. The amount of the premium deposit absorbed during the taxpayer's taxable year shall be determined in accordance with the schedule of unabsorbed premium deposit returns in effect for the company during such taxable year. If the taxpayer is unable to determine the applicable rate of absorption in effect during his taxable year, he shall compute his deduction on the basis of the rate of absorption in effect at the end of the company's taxable year which next preceded the end of the taxpayer's taxable year. In such a case, an appropriate adjustment will be made upon the final determination of the rate of absorption applicable to the taxable year.