Coolant temp gauge working?
I usually fill the system up, run it for 5-10mins and keep an eye on the gauge. Turn it off, then take the bleed screw out, when water starts coming out I put it back in and top the header tank up to suit. I then run it for 10min+ until the thermostat opens and you feel the whole radiator is hot, the fan will cut in and the coolant temp gauge will sit at the same level happily...
Ive always done it like that with the 172 engine and its been fine...