AdrianC.:
Never took sky flats, only panel flats. I usually reduce the illumination so the flat will have up to 5-6 sec exposure. Don't need bright source that could cause reflections, but even illumination. Perhaps that is why some report better flats with sky flats ...

That's interesting. I take my flats at 10% brightness to get 2 second flats, but I will try the lowest brightness possible and see what happens.
Edited ...
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