Chris,
It's not necessary to deal with bias frames at all.
My understanding is that the longer exposure used to obtain a "good" flat requires its own dark to remove any/ all dark "thermal noise" related issues.
AstroArt Manual, p167
""But which role has the bias frames in our CCD Image processing? Each frame captured with a CCD camera contains the bias frame. Then, when we subtract the dark frame, we are subtracting at the same time the bias frame and we do not need to deal with it any more""
(The bias frame only comes into play when attempting to scale the dark from one exposure and/or temperature to another)
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