From previous link the ADU percentage description:
ADU Percentage: Analog to Digital Units (ADU) are the dynamic range of pixels values in your CCD. For a 16bit CCD, the maximum possible ADU is 65535. The actual maximum ADU depends on your CCD. ADU count is proportional to the electrons excited by incident photons and CCD gain. Set the desired ADU % and Ekos shall adjust capture time until target ADU % is reached. When using this setting, it is recommended to set the exposure time to 1 second and Ekos will adjust it accordingly.
There is a lot of information out there about capturing flats. It really depends on the camera. You want a good signal, but do not want to saturate pixels. My 1600s are not true 16 bit cameras, so do not have full 65535 well depth available. Even though they call them 16 bit, they are actually 12 bit with bit shifting, so this affects maximum ADU.