https://www.youtube.com/watch?v=Kkg441UBNpo
check this video out (there are actually 3 i think) - it isn't necessarily just about the ADU with the modern cmos, they alter their read out if they are captured quickly (so adu and length of time to capture the adu). Perhaps try and dim the flat source and take a longer flat to get to the required adu. worth a try.