People rely too much on these things without understanding how they actually work. So just be wary and do go back and recalculate periodically based on where you are now. As your situation changes the outcomes of these things are also going to change. If you're in retirement that may give you a big clue as to whether you need to cut back on expenses or make some other kind of adjustment. Or if you were before retirement it'll give you an idea of whether you are actually close to having enough money or not or have over saved.