Larry Fink recently posted his annual letter to shareholders. He's the CEO of Blackrock, one of the world's largest and most consequential companies. In it, he spoke about retirement. One part that stood out to me was Fink's acknowledgment that retirement isn't just a saving challenge for many, but a spending one, too. It struck me that spending, the other variable of the most basic rule in personal finance (income minus expenses = what you keep), doesn't get enough attention. How do you know how much you c ...