There's a lot of shorthand advice out there, but at the end of the day, if someone wants to get a better sense of what they need to retire, there's no way around it - they have to start tracking their own expenses by categories.

I personally do it every month. I know where every dollar has gone every month, and I also track a 12-month moving average. So I have a sense now of what our yearly expenses are. I compare that with the ever-changing recommendations of what a "Safe Withdrawal Rate" is, and then I try to squint into the future to mix in the effects of: taxes from retirement income; social security (even at the 80% level if no one fixes it); increased retirement expenses from having fun; decreased retirement expenses from not working and getting old; increased retirement expenses from health.

But ultimately, it just comes down to the trio of savings, interest rate, and expenses.