Fill the radiator until the coolant just covers the core. That leaves 2-3 inches of expansion room until the level can reach the overflow tube. It's also a good idea to do the initial cooling system fill through the lower thermostat housing until the coolant in the radiator reaches that level, then install the thermostat and the upper hose and continue filling until you see coolant in the top tank. That totally eliminates the possibility of trapped air in the head.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!