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Doug
Flats will introduce noise. To limit this noise the level of the flat needs to be as high as possible without saturating or in the case of CCD with antiblooming, as high possible within the linear response range of the CCD.
To measure this you need to know when the linear part of the CCD ends.
You can test this by taking increasing exposures of a constant light source and measuring the peak response. Graph this against the exposure time and see where it becomes non linear.
The constant light source can be a non variable star at the zenith.
You then expose your flat so that the brightest part is just in the linear part of the CCD.
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