I used to own a VS Calais...
The Commodore Calais Owners Handbook States:
"The trip computer uses in its calculations three separate calibration numbers. These figures are preset for the standard car, and do not have to be adjusted. However each car does vary due to tyre size, tolerances etc, and therefore the facility is provided to adjust these figures for greater accuracy. See your Holden dealer if you wish to have the calibration numbers adjusted".
Now that doesn't really help I know...
I sort of nutted it out myself as my Distance to Empty, Litres per 100k etc was not right.
From memory I held down both 'set' and 'mode' buttons and at the same time
held down either the 'down' or 'up' button to calibrate (?) take note of any numbers that come up so you have a reference point, from then on its a trial and error business to get it spot on.
What I did was fill tank, drive for say 50kays, fill tank up again, note litres put in off bowser, then check that against what your trip meter says about Litres used.
A little bit of fiddling but it is possible to get it +/- 500ml over a tank fill of 60 odd litres.