Your shutter speed must not exceed the sync speed of the camera. Like Johnnywalker said, shutter speed doesn't really come into play. The f stop controls the exposure on your subject that is lit by the flash(es). The shutter speed controls the exposure behind your subject. The slower it is the brighter your background will be (saying if you're outdoor - day time ). Indoor its effect is minimal, if everything is going to be light up by your strobe anyways.