Except as otherwise provided in this Part, the amount of the taxpayer's section 183 tax that is subject to the surcharge imposed by section 183-a of the Tax Law is determined by multiplying its section 183 tax by a fraction, expressed as a percentage. Where the taxpayer's business activity is wholly conducted in the MCTD, the percentage is 100 percent. The numerator of the fraction is the average gross assets of the taxpayer employed in any business within the MCTD and the denominator of the fraction is the average gross assets of the taxpayer employed in any business within New York State. Gross assets consisting of trucks and other rolling equipment of corporations principally engaged in trucking are located within the MCTD in accordance with the ratio that the mileage of such trucks and other rolling equipment traveled within the MCTD bears to the mileage of such trucks and other rolling equipment traveled within New York State. Obligations issued by the United States and cash on hand and on deposit are excluded from both the numerator and denominator of the fraction. The average gross assets used in determining such fraction shall be the assets employed during the year preceding the privilege period for the section 183 tax upon which the tax surcharge is based.