Like many others have said, I feel like the "4% rule" is a starting point from which most folks can reasonably start from by reading the basic assumptions of the Trinity study and then need to do some research to figure out what assumptions they are comfortable with, what assumptions they aren't comfortable with, and start running some simulations. Between CFIRESIM and my personal hero, Big Ern and the epic SWR Series (
https://earlyretirementnow.com/safe-withdrawal-rate-series/), there's plenty of data and simulations out there for each person to run the simulations that make sense for their scenarios. Hopefully they understand the risk factors, etc. and can plan accordingly. Honestly, Big Erns massive 52 part series has answered any question I could conceivably come up with.
But, yeah, there are some mental gymnastics and "rules" folks may put in place to dumb this data down and make critical decisions more manageable. For me, I really want to leave a decent nest egg for my kids. Maybe ~$500-750k per kid in present day dollars. For sure more than $250k (present day dollars) per kid minimum. I want an absolutely 0% chance of having to go back to work. Zero. I have no interest in barista FIRE or part time jobs or returning to the workforce. I currently make good money in an industry (oil and gas) which is likely to not have good paying jobs for me to go back to when I finally FIRE. Plus my skills and network will likely rot within 2-3 years of leaving industry. And I'd like my simulated retirement to be able to handle being the 2000 or 1966 cohort and basically be 99% reliable for 30 year window AND a 60 year window.
Why 99% reliable? I'm a damned engineer. I can't tell you how many things I've seen break that had only a fraction of a percent chance of breaking and needed 20 things to line up in a row in order to break through the swiss cheese model. Am I overengineering my retirement? Probably. Would I be overengineering my retirement if I hated my job and/or had a physical job that was breaking down my body? Nope, I'd FIRE a lot sooner. So I'm fortunate to be able to make this conservative plans.
Why a 60 year window? I have a number of relatives that lived to 105-110. Everyone in my family lives forever if they don't smoke and get lung cancer. And since I don't smoke, I need to be able to handle living to a ridiculous age. Assuming I lose some weight...
I also give my social security earnings a 30% haircut because Social Security flat out says they will be unable to pay full benefits and there's no reason to assume benefits won't get a haircut since I'm high income and will have high investments and either taxes or a haircut will have to cover this spread- so it's reasonable to assume I'll only get ~70% of the value of my Social Security earnings.
For me, if I plan on a 3.5% SWR for my invested assets + a separate 2 years' cash buffer + having paid off mortgage/paid off cars (yes, I know most folks will say 3.5% SWR + 2 years' cash == 30.6x spending == almost 3.25% SWR) gets me to the comfort level I need in order to be able to FIRE and meet my goals of ~99% reliably ending retirement with likely at least 1/2 of what I started with and therefore easily meet my bequest desires.
I used to think 3.5% SWR was enough for me, however I've started to realize that my final couple of years are likely to be crazy bull market years such that if you look at CAPE ratios and likely initial sequence of risk returns - it's really not unlikely that my first few years of retirement will have substantially bad returns. And the idea of a bequest has become more important to me as I get older. Nothing huge, but something worthwhile. So for me this 3.5% + 2 years' cash (basically 3.25%) makes a lot of sense. And, again, I have zero interest in going back to work in any non-voluntary capacity once I retire. Zero. I did my time as an hourly employee, a server, a computer repair guy, etc. No interest in doing that again.
The difference between 25-27x and 33-35x is probably less than 2 years working time and definitely less than 3 years working time. I haven't fully done that math so feel free to poke holes in it. So for me the extra couple of years is well worth the peace of mind, massive padding, and insurance that I don't have to go back to work.