I love heuristics just as much (if not more than) the next guy, but I’m totally with you on this one Ben. It would be much better to look at your current spending, figure out what will increase, decrease, or stay the same once you’re retired, and take a stab at putting together a retirement budget. Then, compare it to your current overall budget. It shouldn’t be much less than 60% of what you’re spending now, and you’re unlikely to be able to afford much more than 120% of what you’re spending now (if you will be able to, you should be the one teaching all of us :)).