Hope that this helps.

If using the 60deg capability ... you should be seeing a bottom cone diameter of the same width as the depth. So, if you're in 20ft of water ... the circle would be 20ft wide on the bottom (10ft to either side of the transducer, in any direction).

The 60degree cone angle chart would be 1 to 1 (1ft depth = 1ft circle) so it really isn't necessary to calculate it.

You should also understand the shape of a "cone" ... and that the signal on a 20deg cone is only showing a circle as wide as 1/3 of each foot of depth. So, even if you are in 24ft of water ... and the diameter of the cone, on the bottom, is 8ft -- if the fish are 12ft deep, then they have to be hit by the signal at that depth, which is only 4ft wide. And all the diameters of the cone are half on one side of the transducer and half on the other (from a straight line from the transducer to the bottom). That would mean that your 4ft wide circle of signal is only 2ft wide to either side of the transducer at 12ft down. So, if a fish is 12ft deep, but 3ft away from the center line of your signal .... it won't be seen (by a 20deg cone angle).