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Trevor,
The reason why you use ISO-100 is simply to gather information about anomalies in your imaging train, such as dust particles, vignetting and any other aberrations. They are not used to gather noise characteristics.
The dark frames that you take will remove the dark noise for you. The camera's sensor at ISO-800 is going to be much, much noisier than it is at ISO-100. This is why flats are taken at ISO-100.
To completely calibrate your image, you should also be taking dark frames for your flat frames, at ISO-100, because your flats will have noise in them, too. Think of your flats as a type of light frame.
I don't bother with bias/offset frames as I find they introduce noise into my images. When you think about it, sensor readout noise (which bias frames are supposed to counter) is already being captured in the dark frame.
I hope this helps.
Regards,
Humayun
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