For every two hours of billed work, there's an hour of non billable work. There's admin, billing, reporting, business development, etc. that comes with being a contractor ... and that's if you're efficient!
Do you only have one project / client at a time? If so, bill 40 hours and don't bat an eye. Bill at the highest rate you can. My guess is this is the salary at a comparable position divided by 2000 hours ($100k per year is $50 per hour) and add 30% to it or whatever your overhead rate is. If you're talking lawyer rates (~$400 an hour) then yea, you need to bill in small increments and hire out your admin / other stuff to a staff.
Once you dedicate over half of your working hours to a single client / project the rate becomes a comparable employee plus 30%.