You only need to change your offset if your bias frames are either too high (1500 ADU+) or too low (300 ADU-). I tried to get mine as close to 800 as possible so that it doesn't chew into the dynamic range, the higher your bias the lower your dynamic range. If you have it too low however (your bias) you can sometimes end up with negative numbers if you have less than absolutely perfect flat frames (VERY difficult).
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