80% Pre-Retirement Income Mistake
One regularly cited source of this rule of thumb for the replacement-rate figure is AON Corp., a major consulting company. It conducts, with Georgia State University, periodic replacement-rate studies. This is a rule of thumb calculators use that can't do consumption smoothing and thus need you rather than the calculator to come up with a sustainable lifetime spending target. ESPLanner calculates this number for you—that's the whole point of good planning software! So don't guess at the most important variable in your lifetime planning calculations. See also this article from Scott Burns' Asset Builder.