Thanks for the feedback warrenm. Heuristics can be useful, but I built this calculator to help freelancers account for opportunity costs and unexpected expenses in their rates. For example, someone who makes a $100,000 salary would charge 100,000/2080 = $48/hour by the measure you described. However, I don't believe that rate accounts for PTO, National Holidays, Health Insurance, Business Development time, self-employment taxes and many other things. A more realistic equivalency in my experience (and according to the calculator) would be $100-$125/hour.
Hope that's helpful.