The amount of the section 183 tax of a corporation principally engaged in the operation of vessels 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. The numerator of the fraction shall consist of the aggregate number of working days of the vessels the taxpayer owns or leases in all navigable lakes, rivers, streams and waters within the MCTD and in MCTD territorial waters and the denominator shall consist of the aggregate number of working days of all vessels the taxpayer owns or leases in all navigable lakes, rivers, streams and waters within New York State and in New York State territorial waters. The number of working days used in determining such fraction shall be the number of working days during the year preceding the privilege period for the section 183 tax upon which the tax surcharge is based.