Okay, some hard data. I set of with my exploration Dolphin from my carrier with a fresh refuel. The Dolphin has a full tank jump range of 58.49 Ly and unladen of 61.74 Ly. I plotted a route to the other side of the bubble - 6 jumps (I skipped the last jump because I was out of fuel and the jump also was only 9 Ly). I turned off my fuel scoop and started jumping.
View attachment 340329
After the fifth jump I was almost out of fuel and had a jump range of 61.70 Ly:
View attachment 340330
Then I plotted to a system 60.75 Ly away from my current position. I should be able to do that in one jump, but the route plotter plotted two jumps. Sorry for the distortion, it is a VR screenshot, also the route icon is in the way, but you can just about see the kink in the route:
View attachment 340331
Then I plotted the course back to my carrier, still with a jump range of 61.70 Ly. The plotter plotted jumps less than 58 Ly even though there would have been stars in the 60 to 61 Ly range. I turned my fuel scoop back on, filled my tank and was able to jump back without issue:
View attachment 340332
My conclusion: The route plotter does not account for actual ship mass. It
does assume a full tank, and the jump range displayed in the route settings is maybe just for the pilot's information...?