There's an old rule of thumb:
In the beginning, everytime some asks your rate, raise it a little. At some point, you will see the client-to-be blink. You've found the right rate.

Don't means to be flippant but I've found rate-setting to be somewhat of an art. I basically triple everything--the number of hours I estimate a job to take and my billing rate. I first decide how much I think my services are worth--say $100,000 a year. That works out to be $50 per hour. Therefore I charge $150/hour. If I think a job realistically will take 3 hours, I estimate 9. Believe it or not, the combination of this double-tripling seems to work out very well. You wouldn't believe how much non-compensatable time there is when you're self-employed (even when you're very busy). And there's non-paid sick time, vacations, insurance, taxes (self-employment taxes are a doozie), provisions for retirement, slow periods, uncollectibles, etc. etc. etc. The tripling factor barely covers all of that. Working for yourself and making it work is tricky, but it's worth it!!!! Good luck.