Just make sure you don't pitch too low (as Grant Barrett mentioned).
I've been in full-time employment for 7-8 years now, so I don't know what the street-rate for a general-purpose freelance geek is. But you can work out what your personal minimum is, based on how much you need to earn, how much billable work you are expecting to pull in, and how the expenses are likely to pan out.
Then (and again, as the author pointed out), if you find yourself turning away work, raise your fees.
Oh, I thought the article was excellent. I know techs who behave as if their know-how was something to conserve rather than to share; they always make me suspicious that they don't actually have very much of it.
And I liked the advice about fixed-fee and minimum-fee billing arrangements - one of the things that used to stress me out when I was freelance, was the fact that diagnose-and-fix jobs are open-ended; after the first few hours, I began to worry that the client was beginning to worry about the cost. And worrying about that would put me off my stroke.