First few minutes after have started the engine, the thermostat is still close and about half of the coolant quantity becomes warm, reason why the rad coolant level is full.
Next step after few minutes when the thermostat is open, the coolant circulates all around the engine and the coolant level is going down.
In order to avoid a too high coolant temperature and air entries into the circuit through the tank, (air entries supposed to create knockings !!), it's necessary to fill in the tank to compensate for coolant variation ...
I wouldn't take the risk to disregard this detail ...
Our babes are considered as "oldies" now, let's pay attention to the smallest detail !!!