No, the equation doesn't have anything to do with that. Moving money from the highest-yielding checking account I know of with no problematic minimum balance, maximum balance, debit card usage requirement or membership hurdle is equivalent to buying miles at $.0125 before tax, or, depending on your rates, something around $.008 after tax (assuming that, contrary to the Correct View, the miles are tax-free). The risk that your miles will become worthless is something you need to take into account in deciding how much you're willing to pay, or how many you choose to buy; but that factor weighs equally on all possible ways of buying miles. If you're willing to buy lots of miles at that price, then you definitely want to move whatever you would keep in another checking account to BankDirect, no matter what your view is of American's prospects.