When I shot transparency film in tricky lighting, I always based my exposure on a highlight. The lightest area that I wanted detail in I metered and then gave two stops more exposure than the meter reading. Skin tones or similar values got one stop over the meter reading. Start there and bracket and keep good notes. Soon you'll know exactly how much exposure to give for the highlight value you want.
In your sunrise/fog case, I'd place the light fog somewhere in the 1-2-stop over range and go from there.
Best,
Doremus