IMHO knob position is not a good reference for volume. Ideally the volume and tone needs to be adjusted to the situation. So consider room acoustics, how the band is setup, and how loud everyone else is playing. IMHO use your ears to get in the range you want/need and then expect to negotiate a bit with the rest of the band and audio tech.
I find it's useful for me to set up my rig so I can hear well with the volume low. Basically I think it's better to be asked to turn up, than to not be able to hear well enough and still have people complaining I am too loud. So I usually prefer to elevate my cab to about waist level and tilt it back to aim at my ears. This approach can work well if you like a clear and detailed sound. If you like lots of fat down low, you will probably hate it.
Knob position can be a useful reference if you are the type of player who uses a tube amp and likes pushing it to the edge. Plenty of set it and forget players. Some even mark setting on their amp's with a Sharpie.