The precision trolling book is a great reference book that gives you a starting point. If you are really interested in seeing exactly what depths your rigs are running at, then set out looking for a flat piece of bottom w/o structure at your desired depth (ex 80 ft). Start your troll at the desired speed (ex 2.0). Then take the hooks out or bend them so they do not work on your plug or harness and lower your downrigger (this keeps you from getting snagged). When it starts bouncing off the bottom, raise it one or two turns and look at your depth counter on the downrigger. Mark this depth down and compare it to your fishfinder. Do this both into current and with current. Mark these depths down as a reference for later. Do this again at a faster troll speed and other depths. Once you compile enough info, you have your personal precision trolling refernce book for your setup. Sounds like a lot of work, but the info is exact to your style of fishing. No more guessing and trusting others guesses.