"If we only knew when we were going to die, planning would be so much easier. Some retirees believe that it is sufficient and appropriate to base withdrawals on their current life expectancy.... [T]he very significant downside of this strategy is that withdrawals may not keep pace with inflation ... if you have the good fortune of outliving the life expectancy you used for planning purposes when you first retired.... [It] is much more prudent to assume that you will live to your mid-90s (unless you have already reached your 90s) rather than use published life expectancies when developing your spending budget."