If you want rules of thumbs, basically for every 40k a year you need in retirement you need a million. (iow 25 times your annual expenses)

Use that as a rough point, so if you think you need between 100k to 200k in today's dollars (I think that is high because expenses really drop), you will need 2.5 million to 5 million

Also, I prefer calculators that allow for variable spending. I will spend more in early retirement than later.