Chris, when you're in M mode and increase the exposure time to the point of entering LiveComp "mode", if you then press the menu button it will direct you to the exposure time setting for it (ranging from 1/2s to 60s). Initially I set the exposure to 15s because I didn't want too much of blur since the northern lights can move/change relatively fast.
Then I just followed the camera's instructions, pressing the shutter button once so the camera runs 1 exposure while saying "preparing for live composite" or something similar. After that the camera says it's ready while showing whether the exposure would be correct or not in + & - stops. If it was 1 or 2 stops too dark, I'd increase the ISO setting accordingly and vice versa if it was too bright.
I hope this makes more sense.
Nikolas
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Nature produces the most beautiful Art.