Humminbird has an water depth offset setting on it so you can adjust the water depth reading to make up for the depth difference between when the map was made and the current water depth. I would assume Lowrance has the same.
HB has Water Level Offset and Depth Offset. Water Level Offset is used to adjust the map's contour lines to actual depth reading on the FF. Depth Offset adjusts depth reading for distance of transducer from water line or boat's keel.
In the setup menu on the DF. The built in is probably a generic 85/200. You can find that on the Fortrex spec sheet.
I had been running DI on my Hook 7 so I had it set for 85/200&455/800 so I changed the setting to the plain 2D 85/200 and it seems to be fine now.
Believe you meant 83khz instead of 85. The US2 in TMs is a DualBeam 83/200khz transducer.