Every car seems to be different as this article describes how the author added an expansion tank to his MGB to prevent it losing water. Over the 2 years i have never lost any even sitting in a queue waiting to get into our local car show on the hottest day of the year last year.
I sometimes wonder whether they realise that you are meant to have an air gap at the top of the radiator to allow for expansion. Fill the rad to the top and during the first decent journey expanded water will sqeeze out the rad overflow. When the engine has cooled down then the remaining water will find its own level about an inch below the cap. On mine that is where it has remained.
One day if I’m looking for a project to fill some time i may fit one!! See the article / forum thread below for more info about the subject:-