score:4
You're over-thinking this. You're having a conversation with the Immigration Officer. You don't have to anticipate every thing they might want to know and give them all that information the first time you speak. Answer the questions you're asked; if they want to know more, they'll ask you for more.
If they ask you how long your stay is, tell them that you'll be in Belgium for however long it is and then travelling on to Berlin. If they want to know how long you'll be in Berlin, they'll ask you. If they want to know what you're doing after Berlin, they'll ask you.