The amount of investment credit to be added back pursuant to clause ( a ) of this subparagraph is computed as follows:

(1) divide the total number of months of qualified use of the property by the total number of months over which the taxpayer chose to deduct the property under section 168 of the Internal Revenue Code;

(2) multiply the amount computed in subclause ( 1 ) of this clause by the amount of the investment credit claimed on the property to ascertain the investment credit allowed for actual use;

(3) subtract the investment credit allowed for actual use from the investment credit claimed on the property to determine the amount of investment credit to be added back; and

(4) add the amount of investment credit to be added back to the ordinary tax due for the year the property was disposed of or ceased to qualify.