In the context of this article, $50k was my level where I stopped doing work I didn't necessarily "want" to do. Once I had that amount of money, I stopped hunting out for work and let work hunt me out while working on things that mattered to me.

In terms of the larger "for life" FU money, that's one helluva calculation but it's been attempted in several posts on HN I believe. Paying off the mortgage and $50k p.a. for life would do it for me because I'd keep doing things that earned more money anyway. That'd just be enough to live reasonably comfortably if I got ill or whatever.