Annual salary divided by 50 weeks (the number of weeks in a year minus 2 for vacation) divided by 40 hours (the average number of hours worked in a week). Now that you have your hourly rate, double or triple it. Why is it so much higher? That higher rate covers the gaps in a true consulting calendar (it may seem like you're working 60 hours with administrative tasks, hustle, and networking, but you won't be billed for 40 straight hours anymore). You'll also need to account for additional expenses that your employer previously covered, such as health care, vacation days, sick leave, and a 401k plan.
This is your starting hourly rate. If you think it's too low, increase it. If you hire one of the big four consulting firms (McKinsey, Booze Allen, Accenture, Deloitte), there's a pretty big difference compared to a small boutique firm or an independent management consultant. We only spend half of the 2080 hours of work (1040), since studies show that most management consultants spend approximately half of their time on business development and administrative activities and the rest on tasks.
Although this isn't definitive, high-priced consultants often offer external opportunities and benefits that low-cost consultants charge more (or simply don't offer). This is especially true for consultants who don't offer full-time consulting services (at least they charge by the hour), but instead run agencies, a course program, invest in other businesses, etc.