re: 400x your expected monthly expenses for retirement

400x is excessive, but....

You should be debt free to begin with, if you are not this doesn't really work.

You could get away with 250 -300 based on your quality of life, but monthy living expenses, adjusted for inflation and taxes is a must.

There are a lot of ASSUMPTIONS in that as well.

Many already have pensions, SS paychecks, etc. The assumption is that those will be there down the road. That is not a given.