I think retirement calculators are a variation of the old problem: Garbage in, gospel out.
First, they cannot predict the future. The best they can do is to assume the future will be like the past. But what past? Looking back 30 years average inflation is like 2.6%. Look back another ten years, incorporating the late 70s/early 80s excitement and you get a 40 year average of 4.1%. And were the 2000s like the 1980s? And will the dollar still be the world's reserve currency in 20 years?
Second, by delivering multidigit numbers they mislead as to their accuracy. I think 80% is probably the same as 100% and anything in that range could reasonably be reported as "looking good" followed by a page of fine print disclaimers.
Third, by nature they cannot be validated. In the case of FireCalc there would have to be a statistically significant number (1,000?) of forum users dead and reporting their results before you could evaluate the accuracy of its predictions.
So, I think they can be a lot of fun but (with respect to the OP) I don't think the numbers mean all that much beyond very general categories like "probably not," "maybe but dicey," "maybe and optimistic," and "looking good."
Nate Silver, in "
the signal and the noise" tells a story about WWII weather forecasters who were asked for a D-Day weather forecast some months ahead of the date. They said they had no idea. Word came back from on high ordering them to produce a forecast because it was needed for planning.