I would guess he is suggesting that less than 40% means you are over-spending or under-performing and more than 40% means you are under-spending (eg. you should invest more in growth).
The way I read it, it was more like a rough guideline rather than a strict rule... but when you put something forth as a guideline, it does not sound as crisp and presentable. I mean, there is no basis put forward, simply said someone uses that figure when they want to get a rough idea bout a company. I imagine once something passes that filter, they would do a more thorough analysis.