Best way to learn, if you get a signal N-S/E-W, DIG IT.... Dig all constant signals. Low growls are special...
Set your sense where it is stable, usually between 11 and 3, just adjust it till it is stable.....
Set your threshold till it sounds like a faint buzz in your ear.
Set discrimination at 1, and no higher then 2, if you discriminate out poptops your discriminating out lots of gold.
Set volume to max.
In 3.5 years I have never seen a null be anything but iron, and I dug a lot of nulls at first, and still do on occasion when I'm bored.
Good luck and good hunting.......................