If useful life is: The applicable percentage is:
4 years or more but less than 6 years .................... 33 1/3
6 years or more but less than 8 years .................... 66 2/3
8 years or more .......................................... 100
The useful life of any property for purposes of this section shall be determined as of the date such property is first placed in service or use in this state by the taxpayer, determined in accordance with federal income tax law.
W. Va. Code § 11-13D-4