4 KudoZ points awarded for this answer. Comment: "Thank you. Difficult choice! "Norm" is in fact what I had used provisionally, and then it turned out that there was another document (concerning an electrocardiogram) where, in a very similar context, the word норма was used instead of the letter N. So I left it as "norm", though I accept that either "norm" or "normal" may be correct, depending on context. "