I have a pro-troll. In testing its readings, I found it is .075 mV low, so whatever reading it shows you have to add .075 to get the actual reading. So, my advice whichever you're using, scotty or pro-troll, get a digital voltmeter and confirm the readings are correct.
Someone told me that having it wired through a fuse box or common ground can make a difference, ideally you want it wired direct to battery with an in-line fuse. I haven't tried that yet.
Whether I believe in the voodoo or not is a good question. I got it in the "can't hurt" category. I believe if your boat is excessively hot, it can repel fish, but I'm not sure I'm convinced you dialing in the right voltage actually attracts different species. It did help in diagnosing my main motor needed a zinc though, with readings down around .400.
I'm searching for replacement boat right now, and I think when I sell mine, I'm going to throw the blackbox in, and just get braided line instead. At least then you're neutral, and the line is an improvement over steel wire.