The thermostat on the 617 is fully closed at 80°C. and fully open at 94°C. The gauge on the panel will probably climb a bit higher than this figure due to the location of the sensor.

By 100°C. on the gauge, the thermostat is certainly fully open and the cooling system is doing all it can possibly do with the airflow that it has through the radiator. Radiator airflow is assisted by two different fans and further assisted if the owner would clean the fins periodically.