Set you alarm with the fact in mind that it take a second or two to react, and a lone stump or rock on the bottom will NOT always show up...
My draft is 3.5 Ft with the drive down, the alarm is set for 8 feet, gives me time to think a bit rather than ding a prop..
I am also simply not that experienced as a boater only been out there for 5 years going into my 6th so I play it safe