20% is still a LOT to spend on marketing though!
You don't mention how the competition is, but I guess it's quite fierce on short European routes. Personally, unless I had more money than I knew what to do with I'd keep CI around 30. That seems to be a sweet-spot - a kind of 'glass-ceiling' that you can get to quite cheaply but is a lot more expensive to better. I can't see 40%, 40% or 50% making that much differece. You could probably half your marketing costs (or more) and still hit 30.
The old conventional wisdom is never to spend more than 10% on marketing but I'm not sure how true that has remained as the game has developed. As you get bigger, your routes are more mature and you have larger aircraft and multiple frequencies, you'll find relative marketing costs decrease A LOT. It seems to be almost exclusively based on the NUMBER of destinations you serve so medium a/c flying to 4 or5 different places a day suffer the most. As an example, I have 2100 aircraft in my current game, serving 822 destinations and I have 100 CI for one third of one percent marketing!
Advanced Tip: Early in the game, if you DO have the money to spend (unlikely), and the time to micro-manage, you might better spending it on more targeted marketing for new routes. Only the total marketing spend matters, not how or where you spend it so, each time you open a new route, open a route-specific campaign for 2 or 3 months (You decide what level of spend you can afford). That way, your new routes will mature more quickly and you'll still get the CI boost. Just be careful not to let all of them expire or your CI will drop again.