Set aside for the moment the amount the OP has written and let's consider this:
The "tip" is NOT a Fee or a "Supplement" It is a Tip and it's is not required. (I wish the op had posted the actual language from the trip documents)
The printing of a suggested Customary amount of a tip is to help the passengers who have no idea what to tip and are afraid to ask the Tour Director. It also is meant to forestall that conversation as it is awkward and uncomfortable for all concerned.
Another reason for the tip information in the tour documents is that there may be people from countries where it is not customary to tip and they need to be informed about what is customary where the tour takes place.
Tour directors make money in several ways. Usually a small salary based on their days on the road sometimes with a per diem. (And I have personal knowledge of the following) and in many cases like China and Europe and Australia Commissions on what the group buys are a big part of their income.
Let me give you an example from Bucherer in Lausanne Switzerland. Bucherer is Switzerlands biggest chain jewelry store and a frequently visited place by tour groups. tour leaders (like those from Collette Vacations or Tauck here in the US) have accounts with their own credit cards from Bucherer to collect commisions. And here is the part that may surprise many of you. The Tour Director never takes the group to the store itself. On the way in to town to the hotel the tour leader points out the Bucherer store and tells them about the free gift coupon that is in the map they will get from the hotel. They are urged again (for the tenth time on this trip) to use their CREDIT CARDS! for their safety.
The TD gets the group settled in the hotel and his/her next stop is Bucherer where he drops a copy of his passenger manifest off in an office above the shopping level and accessed through an unmarked door near the rear of the store down a side alley.
On the last day in town before departure he may return to Bucherer and other shops where he has also left his manifest and collect his commissions which can be as much as 10-20%. Or at Bucherer they will load his commissions on his card.
In St Moritz a tour leader will distribute his manifest to all the major shops and the ones nearby the hotel. And they will all pay some percentage in commission.
All you need is ONE person to buy a Rolex and your week has just been made...and it happens often.
So Tour Directors/Leaders can and do make some very good money if they are working at the right "level" in the industry a bigger company like Collette. Also if you are at a bigger company like that the job is year-round not quite so seasonal as the good TD would typically do tours on two or three continents.
As for tip amounts or percentages it depends upon where the trip is and how long it is and how difficult. For instance an 18 day Yangtze River Cruise Trip in China could have 5 airline segments in China a boat segment and many bus moves and luggage moves. this kind of trip is high-end and deserves a bigger tip than a one-week jaunt around Croatia.
But what struck me the most about the OP's post is that he seems to be paying 5,500€ for a local one week tour of Croatia which I think is a bit high.
But in any case I think $200 Per couple for that trip would be a very good tip IF the tour leaders were great and you were happy.