Ultimately, your work has to be sold or monetized either by you or your employer. Rarely do we know exactly what benefits our employers get from our work. A helpful starting metric is to estimate what your employer's costs to employ you are. Employers pay for time off, benefits (health, retirement, professional development, etc), and employment taxes that you'll now be responsible for directly.

Folks get hung up on rule-of-thumb advice like hourly rate * 2.5 because they're thinking about their take-home pay and not the cost of employment. In many defense contracting roles, the "fully burdened rate" for contracts is 2.9-3.5x the employee's pre-tax pay. When switching to contracting you can compete by offering better services and by undercutting your (large company) competition -- and that's just where you start. Once you establish yourself as a better performer you can demand a higher price also.