In addition to the credit allowed under subsection (a), a taxpayer may credit against the tax imposed by section
3301 for any taxable year an amount, with respect to the unemployment compensation law of each State certified as provided in section
3303 for the 12-month period ending on October 31 of such year, or with respect to any provisions thereof so certified, equal to the amount, if any, by which the contributions required to be paid by him with respect to the taxable year were less than the contributions such taxpayer would have been required to pay if throughout the taxable year he had been subject under such State law to the highest rate applied thereunder in such 12-month period to any person having individuals in his employ, or to a rate of 5.4 percent, whichever rate is lower.