Since to get the best reading possible from sonar, the signal has to shoot straight up and down. Your correct by taking the position of the boat in the water, which in MHO, is the main reason why people have so many problems with sonar readings. What I did was get my boat out on the water to record how the boat sits well as how it is at a typical trolling speed-for my boat the two measurements were approx. the same. I knew this by taking a bubble level and recording the measurement along the side of the boat and then across the back of the boat. Across the back of the boat from side to side mine was level and this could possibly be addressed by moving gear and properly balancing out your boat. I then reproduced these bubble measurements with the boat on the trailer in the comfort of my garage. (place the bubble level in the exact position you had it on the boat while on the water and then crank up your trailer jack) With the boat now at the same position as it was on the water, all I had to do was mount the transducer so it was shooting straight down and balanced side to side across the back of it. I did this again with the bubble level(helps to have a small one in this case). This method is the simplist I have found and has never failed me. In all the years of running lowrance, I have never had an issue with fish arches, losing bottom at speed and can even see my jig on the graph. Even with all the updates, the only issue I have ever had with lowrance is over heavy cabbage weeds(very dense patches) I have to adjust the sensitivity down and ping speed down to 50%. Currently, with my LMS 332 on the dash, I can mark fish up to approx. 10mph-after that they start looking like a pixel or two.
That’s my two cents and two(.02) more fish bucks!
Jeremy 
