I'm a 'fancy' developer, very experienced in lots of 'in demand' tech (low level, hardware/firmware/drivers, performance, tuning etc). You can get between 70 and 120 an hour depending on the lengths of the contract (and my interest in it). I don't charge by the 'day', I actually do charge the hour.
I would *never* go back to being an employee. When I was an employee, I was 'taken for granted'. These days as a sell-sword people I work with tiptoe around me to make sure I'm not wasting my time on thing that are worthless.
My suggestion? Think of a rate, and add 20%. People will not /respect/ you if you are cheap. I'm totally the same guy as I was before when I was an employee, when my (sound, VERY experienced) advise was ignored again and again because I was part of the walls. Nowadays, if I smirk at some comment in a meeting, people start asking serious questions about the whole thing. Go figure!
Also, don't get robbed by an agent, they will /try/ to get 15% from you, and if the /introduce/ you to a job, it's fair that they get a cut, but make sure that it's on your own terms -- make sure to ALWAYS be on lookouts for other opportunities, and renegotiate if needs be.
I personally think contracting is /the/ way. I'm having a lot of fun anyway, and I've more or less 3* or very likely 4* my net income since I was an employee. However, your mileage may vary -- I spend a lot of time keeping 'sharp' too: no pain, no gain.