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Hi Pete,
The flats need to have their bias subtracted, by either a master bias or a master short dark (sometimes called flat-darks), I never bother with flat-darks since the sensor is at -30C and there isn't much dark accumulation in 10 seconds! Ideally flats are calibrated before being combined into a master when using Sigma or SD combine.
All your images and your cal files have that built-in bias. This needs to be removed in order to scale a dark frame or divide/multiply two frames (flatten an image) otherwise the bias offset will contaminate the division. Darks only need to have the bias removed if they are to be scale to a different exposure length.
Bias frames are the easiest to acquire, there is no reason not to use them, unless of course the camera electronics drift!
EB
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