1. Knob position does not really give you the answer.
1.a) The taper of gain controls varies from circuit to circuit. Distortion in one circuit might start with the control at 9:00, while distortion in another circuit might occur around 2:00. Yet another circuit may remain clean when totally maxed.
1.b) The level you feed into a circuit impacts how much gain you can use before the circuit overloads. If you hit a circuit with a weak signal, you have to turn the gain up; you might not get distortion until to control is above 13:00. If you hit the circuit with a hot signal, you have to turn the gain down; you might have to turn the control down to 10:00 to keep the signal clean
2. The way the signal transition varies from circuit to circuit. I think of it as four stages, but this is an oversimplification.
2.a) The signal has full dynamic range.
2.b) The signal begins to compress.
2.c) The signal begins to distort (OD)
2.d) The signal begins to distort heavily
How quickly a circuit transition can vary. Some circuits have a very narrow transition window. The signal goes almost immediately from clean and dynamic to heavy distortion. Other circuits have a wide window and transition gently from dynamic, to compressed, to OD, to heavy distortion. How the circuit behaves is dependent on designer intent. In some tube circuits the characteristics of the tube can have an impact as well.