3x salary is actually a great formula for equivalent compensation.

If you make $30/hr salary, you really make $40/hr with vacation/health/employer-paid taxes/use of office and equipment for free/reimbursed travel+conferences

+you don't get paid for slow times, and non-billable tasks

+you have to compensated for the increased risk you take on (consider the reverse--you would take less average pay to know that it's completely reliable and consistent)