[size=medium]as I use for virtually all discriminating models in trashy sites:
Sensitivity: As high as possible w/o chatter.
Threshold: (More properly a 'Tuner' control) Would be set for a slight audio (at the threshold of sound).
Ground Balance: Usually, but not always, I would set it as close to 'spot-on' as possible to just slightly positive in the All Metal/Pinpoint mode. However, this can vary with the particular make or model used, and the site I am searching.
Discrimination: Generally, I set mine just at the point where I just reject a common iron nail on top of the ground. There will be times, when in extreme mineralization, for example, that I might adjust lower than that, even to the point where I am in an All Metal Accept setting (on those models which provide such adjustment). Seldom, very, very seldom, do I ever use more discrimination than iron nail rejection because it can encroach on small, thin gold jewelry (like the little 14K ring I got yesterday), and it can impact depth of detection, and add to good-target masking.
I would use a 5