There isn't much of metagame if number is only in Ballmers mind. No matter what guesses you choose he can force you to make at least log_2(100) guesses. Doing anything except splitting in half will only increase amount of guesses. There are two things that can change the game, requiring Baller to write the number on a piece of paper before the start. Other thing you could do is writing a number on piece of paper yourself halfway during the game. If opponent is changing the number adversarially with goal of maximizing guesses you can force them to "pick" a specific number. Afterwards you can open the piece of paper and claim that you actually guessed the number with the first attempt.