On my canon i had to overexpose to get descent flats, 2 stops
ADU of canon800d sensor is max 16384 if am i not mistaken , you need flats about 7500 to 9000 ADU, roughly 40 better just below 50%
So you need to know what is the ADU for your camera .
That is what i learned sofar.
i made them using a computer screen, started up paint wich present a white background....put the scope as close as i could..
Gave up the white T-short method...
I used an iPad with a white screen for my first flats and got horrid vignetting in the final image. Then tried diffusing it with a T-shirt and it doesn't feel very reproducible.
How are you checking the ADU values ? I am guessing Pixinsight has a way -- going to look into that