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The pefect flat should be just that-perfectly flat.
The reality is that there is usually some light drop off towards the corners of the frame resulting in the dimmer corners seen on the flat you describe. The average histogram for the entire frame should be about half the total possible for your sensor. It should not be much more than that as the antiblooming feature (if present) will make the sensor response non linear above this level and the resulting flat won't be as accurate.
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