I should point out that I'm considering the 2X to be the total operational costs to the employer for having an employee. In other words, if I am billing a client $X per hour then my expectation is that my salary will never be more than $X/2 per hour.

As kasey_junk points out, an employee will have federal income tax, social security and medicare payments as well as health insurance premiums removed from their paychecks. Some employers also offer additional insurance policies such as short term disability, long term disability, and life insurance. Those are optional and would be additional deductions from the employee's paycheck.