As said by Grunt previously on Delco, odo is calculated pretty much the same.. eg. 100 000 + 1440 = 101 440... divide by 16 = 6340 .. convert to hex = 18C4 ... then FFFF - 18C4 = E73B
I'm still a little in the dark as to how people determine from these data dumps, what is operating instructions, and what is stored data.
Its all a matter of a systematic trial and error approach to discover where certain data is stored. Odo would have been found by simply grabbing a dump then driving a few kilometers then grabbing another and comparing the difference.
When you say a "modified fuel tank" do you mean you changed the fuel sender or something? as changing fuel sender will change the Ohms reading sent by the sender to the cluster.. hence cluster may need to be calibrated for the sender to provide correct readings.
VY/VZ clusters store both the ohms value and the corresponding tank volume, I reckon that these do the same. Same with lpg values,speedo,revs calibrations ect.