The reason that I like the calculator linked in the MMM article much more than the Vanguard calculator (and most other online retirement calculators) is that it comes from an opposite perspective.
Most retirement calcs: ask you to input the age you want to retire and some financial information and then tell you if you are "on track" or not. As pointed out, many of these have silly arbitrary limits and assumptions which make it difficult to even model my true situation.
The one at networthify linked by MMM has you put in your current savings and living expense numbers and tells you how many more years you need to work. For me, that is a much better way to look at it. It could be made more complex, of course, but the simple inputs seem to cover most bases.
Between the one at networthify and firecalc, the bases for retiring early and safely seem pretty well covered.