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I agree with Grant. You want to get your cal frames right before you go any further. This is really important for something faint and fuzzy like a comet. I tend to take 50 each of flats, flat darks and darks at the same temperature/gain/ISO. You need to make sure that the flats are taken at the same focus and aperture settings. I expose the flats to have a median of around half the saturation level. All of the cal frames are then averaged before you use them. The calibrated subs are then:
calibrated sub = (light – dark) / (flat – flatdark)
I tend to use sky flats but I do have a cheap artist’s tracing panel that I bought from Amazon which works well if I can’t get a sky flat.
One other thing. For Bayer colour sensors I tend to debayer the (flat-flatdark) to extract the green channel before then using it to divide the (light – dark). This is because I tend to use sky flats which are generally blue and which would therefore change the white balance. There are lots of ways to do this which ultimately work the same way but the key thing is to start with high quality cal frames by averaging lots of them.
In terms of what ISO to use with your camera have a look online at astro website for your particular camera. Assuming that you are using raw format there is an ISO which is generally optimises read noise and dynamic range. An example for the Mark 1 Sony A7s is here: http://www.astrophoto.fr/sony_a7s_measures.html.