These are all expenses that have nothing do with being a contractor vs an employee. Only some of both category will need them.
2.5x is the golden rule. You will need to double the salary to cover additional expenses (some of which weren't mentioned, like employer's side of FICA in the US, time spent on sales because contracts end, legal, etc.) you will incur. The .5 of that equation is your profit. Because you're running a business.