If one of my J4 Hercules jumps 2 parsecs, it could drop off 838,710 tons of fuel in pods and have enough fuel on board to get back to their origin point. Getting that fuel out there takes the 680,000 tons of fuel. They could create fuel dumps that way.
EDIT: Acfually, it would be different once the fuel pods were dropped. Or even a partial load. I’d have to actually mess with the sheet to be sure. But the ship is made to jump 4 with 900,000 tons of pods. It might be able to J4 out and only take some of the fuel to get back, leaving a good amount.
Edit2: 360,000 tons to J4 back, leaving 478,710 fuel behind.