I used this cost calculator a year or so ago and found it very helpful. It helped me decide to include things like advertising, gasoline, estimating, and vehicle costs in my hourly rates.... having an idea of where my money goes, what I am charging for, is a really good thing.
You have to, or not your earnings will quickly go to zero.
That's something that took me a while to learn. Billable hours shouldn't just be the time I'm actually pulling wire or bending conduit. Travel, gas, time estimating, time recruiting, time marketing, time spend printing marketing material, time spent with the website guy, etc have to go SOMEWHERE. What's hard is knowing where each of these go