In the case of taxable years beginning after December 31, 2006, the Secretary shall adjust the $15,000 amount under subparagraph (A) at the same time and in the same manner as under section
415
(d), except that the base period shall be the calendar quarter beginning July 1, 2005, and any increase under this paragraph which is not a multiple of $500 shall be rounded to the next lowest multiple of $500.