Incorporate run-down of assets, which would reduce required assets substantially.
while it is nice in theory to be able to live entirely off of the returns your portfolio generates, it is impractical for 99 percent of the population. Most people will need to plan on some run-down of assets in their later years, which substantially reduces the assets one needs to retire. If you want, you could make your calculator include a "desired bequest" amount in order to preserve some capital for future generations.
Adminkablamo (Admin, Networthify) commented
I realize that my ideas are not the mainstream and this reguires a shift in pov. Let me attempt to persuade you.
One problem with spending down all your assets is you can't predict when you will die. On average, Americans live until about 80. But thats the average. Half of everyone will live longer than that. What will you do if you live to age 100?
If you retire at 50 and live until 80 you will spend 30 years in retirement. Thats a long time. Trying to predict the exact moment you will die and budgeting just enough for that is impossible and risky.
You need to be thinking like a B-list celebrity. You made a small pile of money, but your career is over and no one wants to hire you anymore and you need to find a way to make that money last another 30 - 50 years which will probably include a big recession or even a depression or two. The way to do that is not spend down your principle.
Also there are definitely any places in the world where this might be difficult, but I don't think its out of reach of most people who make > $50,00 a year and live in the US.