Captain Eric covered your question pretty well.As for setting a lure at a depth to try and catch fish showing on your sonar.Your sonar unit may be a foot or two below water level when mounted at bottom of boat so the fish marking can be a couple feet deeper than marking if directly below boat.But if fish off to the side will be shallower than showing.So it is a range.Fish in clean water can come up 10ft or more to grab a lure.It is less in dirty water.That is why you want to cover the water column initially as capt Eric said.