FIDE tournament rules: Is a player allowed to scribble notes on the scoresheet while the game is in progress? Do FIDE rules allow a player to make notes on the scoresheet like 


  Here, I wanted to play c4, but thought it would be better to play d4
  first.
  
  He played Nc6. I think he should have played Nbd7.
No. Notes could help you during the game, and therefore they aren't allowed. The Laws of Chess say:

12.3.a During play the players are forbidden to make use of any notes, sources of information or advice, or analyse on another chessboard
[...]
12.4 The scoresheet shall be used only for recording the moves, the times of the clocks, the offers of a draw, and matters relating to a claim and other relevant data.