Since I do not know your skill level, location, and other variables its hard to say exactly. Thats why I suggested comparing to what you could earn if you were to take a full-time job in your area, at your experience level.

Also you said you have worked on fixed price projects before. If you know the scope of the work, and you can estimate what it will take you to get it done, you could charge 1.5x what you think it would take for you to do it fixed price then brake that down into an hourly rate. The extra .5 for changes, and miscalculations on both yours and clients side.