No, you are right. It's a tool for use to compare from one situation to the next. Since your setup while wheeling changes (actual CoG) and whatnot, it doesn't have to be perfectly accurate. It's just something to glance at and give you an idea of where you are in the grand scheme of things because everybody's pucker factor differs.
I was simply mentioning that further you get from vertical in the mounting, the less accurate it will be. So much so that at one point, it will give you a much lower reading to begin with and then shoot to a high reading without venturing very long in the middle.