Charge for the value you deliver not the time it takes. E.g. you increased the client's sales by $500,000 for a net profit of $100,000 - charge $20,000. You saved the client $150,000 by doing something differently - charge $30,000.
When you consistently deliver measurable value to clients, you won't be staying in a full-time job for long. Consulting will be far more fun.
Run-of-the-mill dev work is not it.
I have had many jobs where they ask "How did you impact the business bottom line?" during annual review process.
Usually the only impact will be noticed at least a year after the evaluation.
Most of companies who are considering a consultant are organizationally incapable of telling you how much money is saved or generated. On the other hand, most companies have a sense for how much an hour of work should cost and would be comfortable discussing the project in those terms. So you'd seriously be limiting your options if you focused on the former.
in my experience this is based on nothing and might be based on what HR thinks which is irrelevant