I think this is a pretty good ball-park. $6,000,000 diversified stock and bond mutual funds with 60-80% of stock with a 4% annual withdraw rate adjusted annually for inflation gives you $240K yearly and the Monte Carlo gives you about a 95% chance of never running out of money. Of course if you are happy with $120 yearly, $3,000,000 will do it. Basically, every
1,000,000 gets you $40,000/yr.
These figures are generally considered "main stream/reasonable". The Dave Ramsey %14 per year withdraw is an outlier that virtually no one else endorses.