I'm only running a local small-scale outfit, with a single competed route, so I dont really need a massive CI anyway : but keeping marketing spend at about 10% of revenue did push it up to CI50 rather painlessly. I then opened a base and that really lit a fuse on it : spend went to about 15% and CI flew up to 70 in no time at all. Would have to factor in the big batch of RI spends on the new routes as well though. But even when they ended, the CI stayed around the 70 mark fine.
9/11 did throw things out, I cut marketing spend and even putting it back hasnt got CI above 60 since. Which is fine for my operation, getting it any higher would only be an ego thing : nice to do but I'd rather spend the $$$ getting the fleet in shape to last until game-end.
I think the way to go is add campaigns as you grow, keeping your old ones running and let the 'inflation' increase their spend : and, avoid cancelling them : maybe using temp campaigns is the way to control marketing spend, renewing or not as funds dictate but not hurting your underlying gains from early cheap spending?