I only do this stuff for fun, but I have been running my own Software Development business for 6 years now so I know a little about running my own business.

When you work out your Hourly rate you need to remember that you need to factor in more than just the time you spend Modelling, Texturing etc, because you also need to Invoice them, Quote on Jobs, liaise with the client for final concept buy-off etc, which all take up your time. So just remember you do a lot more than just design.

Your hourly rate needs to reflect every thing you do.

Good Luck
Paul