455kHz is 2 beams (a left and a right) at 86° each and measured at -10db each ...
It’s important to understand that the sound pulse travels past the 86° measured angle ... and diminishes in strength continuously. So in actuality each beam (left and right) covers from the water’s surface to past vertical under the boat (this is why a target directly under the boat can sometimes be seen in both the left and right SI) ...
And it’s also important to understand that there is no “industry standard” of what -db measurement is used ...
Some brands measure at -3db (half of the sound pulse strength) ...
HB measures at -10db (I think this is 1/3 of the sound pulse strength) ...
Rickie