What rate to charge for providing iOS Development consulting work?

I'm a mid to senior level iOS Developer. I have had about 3 years experience in iOS Development and 4,5 years in .NET with 3,4 apps in the app store and developed a few apps for clients/work (fixed price). I'm taking on this contract paying hour rate for the first time. I'm just wondering how much I should charge per hour?

I'm located in Chicago, IL.