each ship can cover 270 degrees. So they should be able to form a 360 degree death sphere pretty easily.
Why would you assume the Daemonship is the capital ship?
I couldn't find the newest version of Battlefleet Gothic: Armada. Or to be more accurate I found one that was from 2003.
Anyways why couldn't they cover each other's rear arcs with 4 ships? One ship correctly positioned can cover the rear arcs of two other ships.