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The QHY8 and 8Pro were not a big problem with flats but the 9 was so sensitive I had to use a few sheets of A3 white paper to get the luminance down so I could use a reasonable shutter length exposure when using the EL sheet.
I also aimed for an ADU level around the 20000 mark as the camera itself only has a well depth of 27K while the images you are correcting have already passed through the Analogue to digital converter and are in effect 16 bit images with a max level before saturation of 65K. I also found that the flat must be corrected with a bias frame or at least a flat dark in order to get them to work correctly.
Give it a try and see how you go. As for focus, you must have the focus, camera adjustment and every other thing in the imaging train in exactly the same position to get a set of flats that will work. It is always much easier to take them at the start or end of an imaging run to ensure all is the same. With a mono camera you have to take a set for each filter but just one set of bias files is enough.
Flats are a pain in the backside but they do make a huge difference.
Good luck Trevor.
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