I suggest starting by setting a bare minimum for yourself – the rate you need to charge to survive. Freelancing should probably be about double the rate of full-time employment, as you have a lot of unchargeable (or just plain idle) time, so you have to allow for that.
Then, it's really just a matter of looking around and trying to figure out what other people are earning for similar work. It also helps if you have some idea of the value you're providing for your clients, as your rate should be roughly proportional to that.
For instance, I know one of my main clients charges out (both my time and theirs) at a flat rate of NZ$150/hr, and a bunch of their time is unbilled, so the NZ$80 they pay me is in the "reasonable" zone (especially for pretty solid guarantees of ongoing work). My "survival" base rate would be about NZ$60/hr (NZ$40/hr at the kind of hours I manage to bill wouldn't really provide me a living.)
But like I say, you're in entirely different circumstances to me, so the details don't really translate at all. It's a matter of (for all of us) figuring it out for yourself, really.