And I now realise that you're using a different assumption about the size of a jump drive than I was. I always assume as a default the HG/MT/TNE/T4/GT/T20... one of the jump drive being 1+Jn% of hull displacement.In the example you are using an engine with J-6 potential, to get the fuel savings. IE the ship is over-engined, so the drive is 25 Tons bigger, but the fuel is 10 tons less. Ultimately a net loss of 15 tons of cargo.
That makes a Jump-1 using drives of various potentials but the suggested reduction in fuel requirement always require 12% of a ship's volume.