if you look at the plastic tank that is attached to the rad' it has min and max marks (ehh, i think), when you top up the coolant through the radiator it is / should be cold, when the engin is running the pump pressurises the cooling system as well as circulating it. when the pressure exceeds that of the cap fitted it is passed to the tank to fill to the max. when car cools, the cooling system then draws back in the temperarally displaced coolant
97'c + or - 1>2'c is good / accurate, this is when cooling fans operate.
thermostat opening i'm unsure of (i think 70'c > 80'c)