OK, this is doing it from the web. Downloaded and saved the jpeg image, opened it in Photoshop and displayed the histogram. The "mean" of 84.5 divided by the 256 x-range (255 if you wish to be pedantic) ends up at about a third of the x-axis, as estimated previously by using an eyedropper on the background skyfog. Do note, however, that to assess your own skyfog either use the histogram on the back of the camera (easiest by far), or a jpeg camera capture, or a raw that has been converted in nonLinear fashion, as one does for general photography. In astrophotography we normally use LINEAR conversion and that leads to a much suppressed histogram. For other stuff peruse my primary website: http://www.samirkharusi.net/