In 1909, Rafael Guastavino Jr. designed the 93-foot-wide dome of the Cathedral of St John the Divine, Harlem, NY. As the thinnest brick shell in the world, the dome is also a clear example of construction efficiency and material optimization. This paper refutes the common idea that the original calculation notes are lost by showing that the preserved working drawings contain every calculation needed. A comparison of these working drawings with the literature published at that time also demonstrates that Guastavino Jr.’s faster and more direct way to size a spherical masonry dome was highly innovative and original. Eventually, these findings highlight that Guastavino Jr.’s calculation method, its design assumptions and theoretical context are still highly relevant today, for understanding old domes as well as for the design of new ones.