It’s definitely not 40% in finance. The only people that might be getting that big of a proportion in benefits are government employees with taxpayer funded defined benefit pensions.

FICA is 7.65% up to $128k, then 1.45% after that. Health insurance is at most $500 per month (on average). That’s $6k/year, so somewhere around 3% to 12% of wages. Paid time off might be 2 to 4 weeks, so maybe 8% (4 weeks/52 weeks).

I don’t know how to value job security, possible severance, or being able to move up in a company, but it would have to be at least 20% to bump it all up to 40%. US has at will employment, so I don’t think I’d value that too much. Maybe you’re less disposable as an employee than contractor, but does it really matter if you don’t have union contracts stipulating certain processes?

However, I’d still charge at least double the hourly rate of an employed person to be a contractor since you need to cover the risk of not finding future work, but obviously that depends on what your competitors are willing to work for.