Work out all of your outgoings, including setup costs, then split those costs across an average month, and calculate what you'd need to earn to cover those costs & what you need in terms of earnings. You should be able too break it down to a monthly target, weekly target and finally a daily target for earnings.
Personally I find it best to charge on a project basis, although I do have a few clients who provide regular print work for which I bill a pre-agreed hourly rate. I find it more comfortable working to a project cost, I think it just comes down to your personal preference and your clients' preference..
It all comes down to experience, a few times you will quote too little and have to work to hard, other times your quote to much and wont have any work at all . Once you work out what you realistically can achive in any given amount of time factored by your ability you'll soon come to a figure that is right for you and the project.
PS. as a tip, never move on your price unless the client is fully aware that a change in functionality and or design equates to an equal change in costing.
That is, if your price is too much for the client suggest they lower the required spec, rather than you trying to accommodate the client at a lower hourly rate. You'll get better clients for it in the long run.
I charge generally for the job as a whole based on its attributes, such as how long it will take (obviously), its difficulty, responsibility incurred (i.e if its someone elses code and I break it .... I'm in trouble, so this is high responsibility and I am accountable....makes my time worth more) and finally risk.... I will let you figure what that means
I personally find it more comfortable for charge for a job. It means I don't have to keep track of time, and that the client knows what they're getting.
Obviously the downside to that is if you underestimate the time spent you either have to swallow it or go back to the client to tell them you've upped your prices. You'll get some cases that you over-charge and more that you have under-charged but you'll learn with experience and get the idea after a while.