A self-employed accountant once told me that you take your hourly wage and triple it. 1/3 is your salary, 1/3 is for business expenses and taxes, 1/3 is for when you don't have work.
The lucrative part is when you get more than 1 FTE of work and start hiring employees. Which was my experience working for a small consulting firm. I made decent money with my 1/3 salary, but my bosses made $$$$ because we priced for 1/3 downtime but were constantly flush with work.
Good luck!