You can charge for your services in different ways: by project, using an hourly rate, on a retainer, or other methods. The hourly rate is a useful starting point for estimating how much you will charge, if you use a fixed cost or other methods of charging for work.
Don’t look at your salary and divide by the hours you have worked. At your job you do not have to spend half your time looking for work. A 40-hour workweek, and working 50 weeks a year (only two weeks for time off, including vacation and sick days) would give you 2000 hours a year to earn a living. You can count on spending 1000 hours a year looking for work, writing proposals, and other activities that do not bring in any money. Then you would have no benefits, so you would have to pay for them yourself, and you will have overhead expenses. Therefore, you will need to charge at least 2 and a half to 3 times the hourly amount you would expect to earn in a regular job.