I gave this a little thought, but have not decided to consult yet. I would shoot for at least 1.5x your current hourly pay, and 2x would not be outrageous. My company breaks out bennies in dollar terms, and they claim 1.35x my salary to include all the things you now have to provide yourself, like FICA taxes, disability insurance, vacation time, health insurance, 401k. You also get to pay for or do your own accounting and other corporate functions, now that you are a business.
If selling your services is easy, as it may be in your case, then settle for the low end. If you need to spend 20-30% of your time drumming up new business, say you want to branch out to other companies, go for the top of the scale. All the sales and contract negotiation time is unpaid.