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ISO and exposure times are largely irrelavent (within reason). Too short, and shutter artifacts can intrude, Too long and you need to dark subtract the flat.
The lower the ISO the better noise wise, its a completely seperate image to the main image, the flat settings are not related in any way, unlike a dark . Just set ISO and exposure times to get say a 0.1-2 sec exposure to a third to half of the histogram at the same focus setting of the image. The ambient temp is also irrelavent for such a short exposure. Colour temp of the light scource can affect the histogram of each RGB sub, but if you flat each RGB sub with its own RGB flat (histogram level corrected) then thats OK. A neutral flat light scource makes things easier, but can be hard to achieve. Dusk/dawn flats are the best way, but again hard to capture.
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