Well I think you mean the thermostat, which opens when the engine reaches a certain temperature allowing coolant flow to the rad.
Frankly, it seems that everything is working, and the normal temp of your car seems to be left side of 'L', presumably this is where it points during normal driving situations. This doesn't leave much room to the danger zone, I know, but as I said before the Zetecs seem to run hot.
I think I would just get used to the car at this point since now that you know the normal swings of the temp needle you should be able to better determine when something abnormal happens.
Happy motoring.
p.s. Having said the foregoing I do not understand why temp drops so low with a/c on, however lower better than higher in hot weather.
