Note the inherent flaw in "by the hour" billing: if you want to increase your income you either must work more hours or raise your rates. In other words, hourly billing guarantees a built-in cap to your annual income. You have a maximum, but no minimum income. This is a world that I do not want to live in: a voluntary upper limit on my achievements. Hourly billing is a method of setting prices by looking at the cost…
It might sound strange, but most service providers are "selfish" in their pricing. Meaning, a rate is determined by looking inward — how much do I need to charge to make more than my monthly fixed expenses? What's the equivalent hourly rate when I was a salaried employee? What are my peers charging?
I've been able to continuously raise my rates over the years ($50 -> $250+/hr) by getting better at understanding the business problem at the root of a project request (i.e. they don't "want a new website designed", they really just want more walk-in customers) and tailoring my proposal, discussions, and execution toward that end.
As long as you can position yourself as an investment instead of an expense, and wield your technical + business skills to make your client's business better off than they were before hiring you, your ceiling is whenever the project cost outweighs the potential payoff.
(FYI, I wrote a book on this: http://doubleyourfreelancingrate.com)