The Q-range corresponds to the maximum momentum transfer at one edge (not corner) of the image, assuming that the beam zero is in the center of the image. It depends on the camera length; if the camera is far from the detector (typical for small-angle scattering) the Q-range will be small while if the camera spans a wide angle relative to the sample it will be large. 2-theta max and the Sample-Detector distance are automatically updated every time the Q-range is changed, and vice versa. In addition to setting the q-range you should also verify that the wavelength is correct for your instrumental configuration.