Rob to take an image of your histogram, open your eos viewer, navigate to the folder of flats files and double click on one. A new window will open with an enlarged version of the thumbnail along with a histogram on the right. Hold down your press alt+prtsc keys. Open Photoshop, open a new image (should come up with clipboard as the default size) and paste. save as jpeg and upload.
Heres one I did earlier
This Flat is over exposed slightly. Aim for 1/3 from the left of the histogram edge.
Also take note of what Mike has said about moving your camera, flats must be taken at the same orientation, focus and I believe ISO but that's just me
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