How do you know how much to charge per hour of your time? The idea is that you should earn approximately the same amount of money by whichever billing method you are using. It doesn't make sense to price yourself cheaper by one method than the other. Thus, don't set your hourly rate based on someone else's rates, but on your own.
How to calculate your hourly rate? How many words can you translate per hour in an average text (mid-way between the hardest and easiest texts you typically translate)? Don't forget to account for time spent on research and proofreading.
To help you get an accurate idea of what to bill, try tracking the total time spent for a few of your typical translation projects. Divide the total earned by the exact time spent translating, researching and proofreading to find out how much you earned per hour of work.