You have to know the location of the depth transducer AND the keel info to set the depth offset correctly. The easiest way is tie a weight to a string. While in the slip drop the weight over the side and let it down till it just touches the bottom. You should be athwart ship of the depth transducer also. raise the weight and note where the line stops being wet. measure the length of the line from the water mark to the weight. That is the depth at the transducer.
Depending on how you want the depth meter to read adjust the offset so that it correctly reads the depth below the keel or the depth of the water.