Line leading down is important. My rod holders set higher, and rod tips are higher than my neighbors so If I get a "hot" feet back reading from them, I usually add 4' to start, then adjust. We usually run a 3.5 setting rather than a 3 to get max distance from the boat, this is listed not on the charts or trolling bible so a little extrapolation is needed. A 1.5 setting is pretty easy to set, somewhere between 1 and 2 on the chart. If 3 people fishing, I run a 3.5/1.5/0 setting.

A neat test over a mud bottom is to be in say 20' of water and release line slowly until it starts bumping bottom, then you now know where 20 feet is in relation to your line out. (use that old chrome and yellow dispsy just in case)