In the US 2x or (100%) is the equivalent to a direct hire salary. Since the employer taxes, social contributions, and paperwork filing is directly managed by the contractor now. You’ll have to be your own HR or hire someone to do it for you.

That's not really true:

-- you can tax deduct a lot of things, of course, which include day-to-day travel, equipment (phone, internet, laptop), and much more. A part of rent. Let's not even talk about the SEP IRA you can use to further put away money for yourself and lower your tax burden. Unlike an employee 401k, you can decide to put a large chunk of your salary in an IRA at any time, including up until Apr 15 of the following tax year.

-- I'm about to find out, but the lower corporate tax may be a boon here

-- You work 40 hours, or whatever hours. Employees work INFINITE hours. Weekend work, on-call anyone?

I'm not sure what you mean by own HR. For healthcare, you can use the state marketplace or you can look into things like Freelancers Union. For paying tax, you have an accountant. For naughty behavior to report to HR, you have only yourself...

3x is the norm for short term it contracts in the UK - ok if its a year you could go lower.