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..