There's always the question if the findings in Gangelt are applicable to other places in Germany, never mind in other countries. I think it should be taken with a grain of salt as different countries and even different regions within a country will have a different fatality rate due to numerous factors. However, I believe that it's also similar to the fatality rates reported in South Korea and Denmark, and even on the Diamond Princess; to me that just confirms that the virus itself is not as scary as long as the hospitals aren't being overrun, hence why lockdowns and other containment measures matter a lot until there's a vaccine/proper effective treatment . That said, Spain's number of actual cases are definitely gigantic either way, I do believe that your estimates for Madrid (or Italy, or any other country with high death count) are not far off. This is an interesting study from Imperial College London. Their estimates are approx. 6 million infected in Italy and 7 million in Spain (as of March 28th). https://www.imperial.ac.uk/media/imperial-college/medicine/sph/ide/gida-fellowships/Imperial-College-COVID19-Europe-estimates-and-NPI-impact-30-03-2020.pdf