"[R]oughly half of those who enrolled in the ACA's exchanges and received [Advance Premium Tax Credits (APTC)] ended up owing money to the federal government, because their income ended up higher than they had anticipated.... One solution is for enrollees to wait until tax filing to receive their credit as a lump sum. For most people, however, that would diminish their ability to afford monthly premiums. Another is to claim a partial amount of what they are eligible for at the time of application. This would build in a financial cushion[.] ... All [but one of the] exchanges set the default amount of tax credits that applicants receive at 100 percent of what the exchange determines them qualified for, based on previous tax data and the applicant's prediction for the coming year.... DC opted to take a different approach, lowering the default to 85 percent."