(a)qualifying assets begins to be used for purposes other than the tonnage tax business, an appropriate portion of the written down value allocable to such asset shall be reduced from the written down value of that block and shall be added to the block of other assets as per the following formula:—A = B x (C/D)where,––A = the appropriate portion of the written down value allocable to the asset which begins to be used for purposes other than the tonnage tax business;B = the written down value of block of qualifying assets as on the first day of the tax year;C = the book written down value of qualifying asset which begins to be used for purpose other than the tonnage tax business; andD = the aggregate of book written down value of all the assets forming the block of qualifying assets;