I tried going through all the math late last night, and got tired and cranky with it, but my one comment on the whole process (dragging chunks of space iceberg into the cargo hold to melt into water to feed into the fuel scoops, is that i'd allow it as possible, but a somewhat long and tedious process. Since the basic formula is that a ship requires 10 tons of fuel per 100 tons of hull, multiplied by the planned jump number, even a modest far trader will need 10 tons of fuel per jump-1. Given that water is 2/3 hydrogen, that's 15 tons of space ice they'd have to process. Bigger ships and longer jumps just make the job bigger. How big is that cargo hold again? :-)