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 of useful life;

(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.