under subsection (a) exceeds the limitation of paragraph (3) for any calendar quarter, such excess shall be treated as an overpayment that shall be refunded under sections 6402(a) and 6413(b). (c) Definitions For purposes of this section— (1) Applicable percentage The term applicable percentage means— (A) in the case of each of the first 4 calendar quarters to which this section applies, 50 percent