In a normal system,the top part of the radiator is called an expansion tank.
The purpose of the tank is to provide space for the Coolant to expand when it gets hot. If you fill the tank to the top of the neck,it will have nowhere to expand when it gets hot,except out the overflow hose and onto the ground.
Not trying to pick a fight,but the proper level of coolant is just above the top row of cooling fins. If you really want to get the absolute most coolant in your radiator,fill it to the top and let the coolant puke out the overflow hose onto the ground.
Keep driving the car,and eventually it will stop,and theres your level.
