Where 12 percent of the total deposits or withdrawable accounts of an institution at the close of the taxable year is equal to or less than the sum of such institution's surplus, undivided profits, and reserves at the beginning of the taxable year, a reasonable addition to the reserve for bad debts as determined under the general provisions of section 166(c) may be allowable as a deduction from gross income. In making such determination, there shall be taken into account (a) surplus or bad debt reserves existing at the close of December 31, 1951 (i.e., the amount of surplus, undivided profits, and reserves accumulated prior to January 1, 1952, and in existence at the close of December 31, 1951), and (b) changes in the surplus, undivided profits, and reserves of the institution from December 31, 1951, until the beginning of the taxable year. A deduction for an addition to the reserve for bad debts pursuant to this section will be authorized only in those cases where the institution proves to the satisfaction of the Commissioner that the bad debt experience of the institution warrants an addition to the reserve for bad debts in excess of that provided in paragraph (b) of Sec. 1.593-1. For definitions, see paragraph (d) of Sec. 1.593-1.