Trying to understand the TVG settings for the FCV 1150. The unit is connecter to a R209 Xducer running at 45 on the low and 170 on the high.

The TVG menu has a depth setting, a numerical setting, and a graph. I'm looking for a detailed explanation of what these settings actually do. TVG adjusts gain to compensate for signal loss due to the combined affect of spreading loss and absorption loss. Spreading loss increases by the square of the distance, the inverse-square law. Absorption loss increases with distance. Does the TVG depth setting compensate for signal loss only within that selected range and ignore targets beyond that range? Most of my fishing is tuna fishing in 300 feet or less. Therefore I'm promarily concerned with individual tuna fish and midwater bait. My experience has been if I set the TVG depth at 300 feet (or the water depth I'm fishing), 1 or 2 on the numerical setting, and low or zero gain I gett too many echoes close to the bottom. I get better results with a TVG depth setting at 2 to 3 times the actual water depth, 2 or 4 on the numerical setting and then increase the gain from zero to get the desired picture. Any suggestions? Can you explain the graph in the TVG menue?