My retirement guy gets asked this all the time. Actually, he gets asked “How much do we need to retire?” and his answer is always the same....
“How much do you spend?”
I find this annoying. What you spend isn't the major problem. The major problem is that you can't count on money to remain of the same value with each passing year.
Since that silly fool got elected in 2008, the price of most of what I buy, doubled. Virtually everything I purchase in my day to day life costs twice what it did in 2008.
What good does it do you if you calculate you will need X dollars per year for 20 years when the real life costs of everything become 2X?
I don't see many people discussing the problem of rampant inflation of money, and no matter how you calculate your retirement, when the cost of things double or triple, most people will not be able to weather it without serious reductions in their standard of living.