I've never used Ektar, but color negatives have much more latitude. If I'm using a spot meter, I tend to rate the film at box ISO. I've shot test rolls showing detail in zone 1 (closing down four stops from meter reading) and zones 9-10 (opening up 4-5 stops from meter reading). I wouldn't be surprised if you could still hold detail past there. Color negs hold highlights very well. That being said, it's generally good to give them nice solid exposure. At box ISO, the shadows in zones 0-2 can look pretty thin and grainy / noisy. When using an incident meter, I rate the negs at one stop over box ISO (e.g., I rate a 400 film at ISO 200).
As for the sunset question, I'd say the same advice applies. There's no formula, but a safe bet is to place the highlights (but not the sun itself) at zone 7, two stops open from your meter reading. Bracket in full stops (half-stops are pretty hard to see on color negs) and see which exposure suits you the most.
Again, the best way to answer your questions is to shoot a few test rolls. You could probably shoot two rolls, bracket, keep notes, and learn what you like, how you like to rate your film, what zone 6 and zone 7 highlights look like, etc.
Bear in mind, with color negs, using proofsheets or small prints to evaluate your results is tough. An individual negative can be printed to look just right, or too dark, or too light, etc. I'd just compare the negatives themselves. See which one has detail everywhere you want it without having blocked-up, too-dense highlights.