Not sure I can help with what yours is doing Simon, but I can tell you about my experiments with mine. I wanted to use the stock VDO gauge, but wanted some sense of accuracy with it. I carefully measured t'stat temps (coolant in pan with calibrated thermometer on the stove top --- when wife was not home). Once I established the actual t'stat temp(179F-182F on my 180 t'stat), I played with a series of 10 ohm resistors in the sender line to try and 're-center' the gauge. It would read only about 1/4 of the way up with the cooler t'stat. With one 10 ohm resistor in the line, the VDO gauge sits just a hair below the middle of it's travel at cruise with the fans off and actual temp right around 180. In traffic with the fans on (set point is 187F) the gauge sits just a hair above the middle of the range.

Don't know if that helps or not -- while playing with that, it seems to me that the gauge reads fairly linearly in the 160-190F range on mine. 82 242 with factory VDO built unmarked temp. gauge.