Cringe -- this is bad advice for setting your consulting rate. In the US, the full cost of an $100K employee is not $100K. It's $100K salary + paid vacation time + employment taxes paid by employer + healthcare + office space & pro-rated related expenses (like cleaning of said office space). This metric is called the "fully loaded cost" of an employee and ends up being 1.3x-1.5x of the salary.
But even this calculation fails to capture the flexibility advantages that your clients get from hiring short term. It ignores the fact that as a consultant you are hired only when you are needed and are 100% utilized, while a yearly employee may not be fully utilized every day of the year. Essentially, an employer that hires a full time employee agrees to "buy in bulk" and pay for time that may or may not be fully utilized. By necessity this means that your rate should be higher to account for the additional value provided plus provide cushion for those in-between project times when you are looking for work or doing proposals.
Two quick and dirty metrics I have seen for determining rate per hour are 1) yearly salary / 1000 and 2) 2x full time employee hourly rate. Both of these converge on about $100/hr. Un-official data from HN seems to confirm a median freelance rate in that range (about $90/hr): http://news.ycombinator.com/item?id=3420203