If you follow any of the FIRE (financial independence / retire early) posts or blogs, a simple place to start is to take your current annual household expenses and multiply by 25. Using that number, if you stick to a 4% withdraw annually you will never run out of money.
Example - if you spend $80,000 last year (house, groceries, car, etc...) multiply by 25.
80,000 X 25 = $2,000,000. You will never run out of money if you stick to a 4% withdrawal annually.
LOTS of caveats and assumptions but it gives a GENERIC starting point that's close to being in the ball-park.
Your results WILL vary.
Personally I've used numerous on-line models and have a financial planner because I don't want to get this one wrong.
I think the 4% rule is misunderstood and now lots of places claim it is something other than what it is or at least what it originally was. As far as I know the original 4% rule said that you withdraw 4% the first year and each subsequent year the withdrawal rate increases to match inflation. This provides money for a 30 year retirement (not forever). It has been back tested and I believe it has always worked but of course past results don't guarantee future results. It also assumes some stock/bond mix but I don't remember the percentages.
The always withdraw 4% and never run out of money rule might work too, but I don't know. In your example do you withdraw $80,000 every year or 4% of that years balance? Neither one seems that great. The former is going to be pretty meager 30, 40, or 50 years down the road. The latter is probably assuming something like a 7% growth in your investments and 3% inflation. That might be true in the long run but short term volatility might be a huge problem.