Justifying your costs by basing what you charge against the costs of a similar employee is a good start, and in my experience it's how most of us start (the old "divide by 2000" trick). I like that Andy makes note that you need to include overhead (prospecting, writing proposals, ...), which a lot of new freelancers tend to miss.

However, I think the big takeaway is realizing that the formula presented establishes a minimum threshold. It shouldn't be used to figure out what you charge. My rates are way north of what the a developer/marketer would command on the open market, but I don't contextualize my costs against the equivalent costs of an employee; instead, I anchor my costs against the upside that a successful delivery of a project would yield for my client.

The single best way to substantially make more money consulting is to stop selling commodity services (web design, Ruby programming, whatever), and to truly consult. Provide your clients with a way to bridge the problem they face with the solution they desire, and charge accordingly.