... what would the starting point threshold setting be?
There really is no such concept - all signals are different, and also, the desired effect you want will be different for some circumstances. The idea is that you learn what a tool does, and where/how to use it. With any form of compression or limiting, the
only possible way to set it up is to listen to the results and alter the settings till they give you what you want or need.
All that the knee setting does on a limiter is to determine the amount of your signal that comes out with zero dynamic range - and this depends utterly on the application. I can think of examples where you'd want to limit this considerably, and other applications where you wouldn't want to do it at all. So where do you want to start?