Ditto. If you can't see the ball on your sonar then you're probably going too fast for anything but smallies...maybe 'bows. At laker-salmon trolling speeds, say 0.5 - 2 mph, I see the ball no problem.

All you need to do is determine the depth from the sonar, check it against the depth shown on the rigger and come up with your own "fudge factor" for getting to the depth you want right off the reading on the rigger. No calculus involved either.

What is the cone angle for your transducer?

The fishfinder only tells you how far the fish is from the transducer. The transducer has a cone of operation, it looks down and down and out to the side of the cone. What you see for depth of the response is true but, it maybe to the side.

Read this article for more information and graphics. A picture is worth a thousand words.

Then how deep is the fish??? (page 9)
http://www.hightechfishing.com/other/Lowrance_Tutorial.pdf