My flats with QHY8 using 2-layers whiteT shirt and clear twilight sky (5.15pm Bris Friday) using Hyperstar + C8.
I shot 10 each at 1, 10, 25, 50, 75, 100, 250 and 500ms. I saw that anything over about 250ms was saturating badly. I recorded the ADU's in Excel and 75ms seemed to be about the best.
Bias was just lens cap on and 1ms x 50 shots. Used neb to do all the bias and flat stacking and master bias subtraction and finished up with a good master flat that left me with a pretty good image after using it. I had previously had the same "reverse flat" effect you mentioned.
Craig Stark has some good tutorials on flats and bias on his website:
http://www.stark-labs.com/help/nebul...tutorials.html. My apologies if you have already seen them.