Ken French says the simplest answer is found in the rate offered on a long-term Treasury Inflation Protected Security (TIPS). You can go beyond the TIPS rate if you don't plan to live forever. Retirees with a shorter life expectancy might choose to consume more. Finally, you might increase your expected investment return and your expected sustainable withdrawal rate by taking more risk. But Ken warns that the expected return and the return you ultimately receive could be very different. That is the nature of risk.
Eugene Fama and Ken French are members of the Board of Directors for and provide consulting services to Dimensional Fund Advisors LP.