I had to force my prior employer to transition me to a subcontract and, including retirement plan contributions, I'm keeping at least 50% more money than as an employee.
My strongest recommendation is to run a full corporate budget -- operating expenses, fringe benefits, tax, etc. That will allow you to objectively see how much more cash you're able to bring in on contract and determine if your hourly rate is appropriate for the additional risk (and work) you're assuming.