I've done a lot of experimenting with this Auto vs. Manual vs. depth. In normal soil, manual is the deepest seeking setting no holds barred, no contest.
Do not automatically unconditionally set manual to 30. Ratchet it up to where you get a tolerable false maybe every few sweeps. This is the manual setting sweet spot for beginners to start out with.
With some experience and perseverance,
myself and likely others here depending on soil/EMI conditions sometimes go full steam ahead, dam the torpedoes, with a 30 manual setting with a false or two every sweep . Yeah its a headache in the beginning but i guarantee it'll get the deepest keepers mostly relying on instinct after a while. With all the hyper pounded parks around here most of the oldies are deep real deep forcing me to go the extra mile and it HAS paid off.
However, i would not recommend this for beginners, it'll drive you crazy.
I rarely use Auto these days but when i do its in extreme EMI areas, tot parks or really bad soil conditions.