I’m always on the lookout for ways to improve the tools on Portfolio Charts, and Siamond really came through with his latest update to the Simba spreadsheet. Buried in the heaps of interesting returns data is something really cool — direct calculations for safe and perpetual spending rates for a given investing period. Based on an equation from a Morningstar white paper, they are particularly elegant compared to my old method and allow me to significantly improve the speed and stability of the Withdrawal Rates calculator. And by doing so they open up a great deal of additional data that was previously too laborious to manage.
Well I’m a sucker for new data, and in the process of updating the calculator mechanics behind the scenes I took the opportunity to revisit an old question I’ve been wrestling with for a while.
How do you calculate a 40-year withdrawal rate when the worst start date for a particular portfolio was less than 40 years ago?