Since I can't afford to put 2 good units on my boat, I bought a second transducer for my Helix 7 SI/DI/GPS/G2 unit. I have it mounted on the TM and everything's ready to go. Say my 'ducer is going to be about 2 feet under the water. Do I need to set my offset at +2 or -2 to account for the 24 inches of water ABOVE it? If I don't adjust, then I'll think I'm in 8 FOW, but actually be in 10 feet.