another 360/408 over heating

Regardless of fully open at rated temp or starts to open at rated temp... all thermostats attempt to maintain a temperature, your home thermostat maintains your house temp to within a few degrees of its set point. Your auto thermostat also tries to do the same. If the radiator can transfer more heat than the engine can produce the thermostat will not be open all the way. If the rad can't, the thermostat will go full open and the temp of the coolant will continue to rise until the engine produces less heat. It is that simple.
If you want to know how your thermostat works get a pan of water and a thermometer and test it.