Set the monitor levels at whatever is needed for your mix. There isn't a "sweet spot" where the audio quality is better. The only exception to this is noise levels tend to increase exponentially as you approach maximum, so I tend to stay away from the very end of the trim range, but the purpose of the level control is so you can adjust the level to taste, and to interface with your outboard gear. If that means the level control is at 8:00, then so be it. Similarly if that means the control is at 4:00, okay.
Does that help?