Ah ok I understand what you are asking (I think).

I have had the volume in the settings menu at the lowest setting in an attempt to get the volume as quiet as possible. This means that reducing the volume using the knob has had no impact on the volume in the settings menu as it is already at the lowest setting.

I will try setting the Nav volume in the settings menu at a higher level and see if turning the volume knob lower changes the volume setting in the settings menu.

The nav. instructions are always at the same loud level.
Changing the volume with the knob only changes the volume of the Nav. System if the knob is turned during the instruction, and only changes the volume of that particular instruction. The next instruction will be back at the louder level

Turning the knob before and instruction or in-between instructions has no effect. It only works during the instruction.

If the stereo is playing at the same time, turning the volume knob during an instruction will lower the stereo as well as the instruction. The stereo stays lowered, unless the volume is increased again after the instruction.
Even if the stereo volume is not increased after the instruction, the next Nav. instruction will be very loud again.