It's not that accurate. It's more of a guessing game if you ask me. "Here's the mark for 15....There's the mark for 20...set it in the middle" Mine set at 17. I put it on the middle or what I thought was the middle.
As far as the knocking is concerned, I haven't heard any knocking but on the same note I don't quite know what a knock sounds like. I've been informed that when you hear it, you will know.