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If you can get your hands on a copy of "The Handbook of Astronomical Image Processing" by Richard Berry and James Burnell it is all covered in great detail (including the math involved). They recommend your darks should be 5 x longer then the longest light sub you intend to take to get the best results using scaled darks. I have been using their method for some time now and it works very well plus it is convienient having only 1 master for each bin regardless of exposure time. Maxim, CCDstack and AIP4WIN let you add the bias to the master at the time of calibration which allows for scaling to take place and this can be done as a batch process which speeds things up a lot.
Mark
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