"Before [the final regs issued Dec. 16], there was no official connection between Section 4980H -- the ACA regulations in the Internal Revenue Code that detail the employer shared responsibility requirements -- and any percentage other than 9.5 percent, which remained the only rate given in ACA regulations for affordability testing.... [The December 16] guidance allows employers, essentially, to [use the inflation-adjusted percentage] for 2015 in measuring affordability even though the instructions and forms are based on 9.5 percent."