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I personally consider flats to be for a lot more than just removing dust donuts. Unlike most on here though, I am also uninterested in doing astrometry which requires a good flat field above all else.
For the "perfect" flat field it must be taken at the same focus as what you are imaging at because light fall off from the centre to the edges changes as the focus does. On a purely cosmetic level, the dust donuts change with focus as well.
As for configuring ADU levels, this is quite easily calculated on camera. Most cameras have a linear response for 75-90% of saturation, this is tested by gradually increasing exposure time (the same way you would normally do flat) and plotting exposure time against average ADU. Typically you want to do 50-75% of saturation for a flat field which equates to 24,000-40,000 but this depends on the camera. As Greg mentioned, have them last at least 5 seconds if they have a shutter.
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