@sebreitz I am talking about 100% correlation, But I agree with you there are nuances in all of this. The principle ideas hold. I did not want to get too academic and start going into Markowitz portfolio theory which I hope I still have a sound grasp of and so won’t use your kind offer of a random calculator on the Internet.
The aim was to get at least some people to think a bit more about what diversification means to them. Even appreciating that it is ‘risk’ that one tries to diversify is a good start. That term ‘risk’ itself has connotations in finance theory that lay people here may not appreciate (and one which I hand wavingly handled).