The term mortgages means loans secured by real property within or without New York State, participations in and securities collateralized by pools of residential mortgages, whether or not issued or guaranteed by a United States government agency, and loans secured by stock in a cooperative housing corporation. The percentage of total assets comprised of mortgages is an amount equal to the ratio of the average of the four quarterly balances of such mortgages ending within the taxable year, to the average of the four quarterly balances of all assets ending within the taxable year. Such quarterly balances shall be computed in the same manner as the Report of Condition required for Federal Deposit Insurance Corporation or Federal Savings and Loan Insurance Corporation purposes, whether or not such report is required. For taxable periods of less than one year, the taxpayer shall compute such ratio using the number of such quarterly balances ending within such taxable period.