I suppose I should know the answer but I don't.
Well, at least you realised that there was a question! A little while before my FI test, I asked a couple of f/w PPL friends if I could practise doing a briefing on them. It was all going well, until one asked, just as you did: "104% of what?" It stopped me in my tracks at the time, as I just couldn't think of an answer that made sense and was easy to understand.