Best ADU for flats?

[Cyberman (Rob)]

Hello. Could someone help me with flats. What would be the optimum ADU for flats for my unmodded canon 600D. I believe that is a 14-bit ADC. So, I think the maximum ADU is about 16000.
I analysed one of the flats that I had been using with ASTAP. It's background ADU was 2467. This seems to get rid of dust motes, but will it get rid of other problems in the optical chain. I sometimes get a halo around a central object in the stacked image.
Thank for your help ,Rob.

[The_bluester (Paul)]

I always aim for the average value of the flat to be around half of the maximum value, with no pixels saturated (Beyond the known hot pixel count) and none at zero (Excluding dead pixel count)