Hello Geoff. I have always worked on the principal that the radiator should be filled to the brim and the expansion tank should be empty so that water is expelled from the rad to the tank. The tank should be designed to hold only the expanded amount of water that an engine at working temperature provides. If you interrupt this by putting extra water in the expansion tank (as you say) then it will be expelled through the pressure relief cap in the expansion tank.
In short, if, with a cold engine, you fill the radiator to the brim and leave the expansion tank empty that situation should be the same when the engine is cold again. If not it is then it is a sign of an air leak or something more significant like a cylinder head gasket.
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