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Which errors in Christopher Columbus’s calculations caused him to underestimate the distance to Asia?

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Map showing the transatlantic and global routes of major early voyages of discovery and exploration by Spain, Portugal, England, and France during the 15th and 16th centuries. The map traces the journeys of explorers such as Columbus, Cabot, Magellan, da Gama, and others, with labeled paths crossing the Atlantic and encircling Africa and South America, as well as lines of demarcation dividing Spanish and Portuguese exploration zones. — European exploration Map depicting the European exploration of the New World in the 15th and 16th centuries, including the voyages made by Christopher Columbus, John Cabot, Alonso de Ojeda and Amerigo Vespucci, Pedro Álvares Cabral, Ferdinand Magellan and Juan Sebastián del Cano, Giovanni da Verrazzano, Jacques Cartier, Sir Francis Drake, and others. The lines of demarcation represent an early division between the territory of Spain (to the west) and Portugal (to the east).

Christopher Columbus’s underestimation of the distance to Asia was not due to a belief that the Earth was flat, as educated Europeans of his time already understood the Earth to be spherical. Instead, his miscalculations stemmed from several flawed geographic assumptions:1

Degree Length: Columbus used the Italian mile (4,847 feet) for his computations rather than the more accurate nautical mile (6,076 feet). This led him to calculate a degree of latitude as approximately 45 nautical miles, significantly shorter than the actual 60 nautical miles.12
Geographic Estimates: He rejected accurate estimates of the distance to Asia, preferring the calculations of Marinus of Tyre and a chart by Paolo Toscanelli, which suggested a much shorter journey.1
Exaggerated Landmass: Relying on Marco Polo’s Travels, Columbus believed the lands of the East stretched far around the globe. He also accepted Ptolemy’s exaggerated west-to-east extent of Asia and added to it, further reducing the perceived distance across the Atlantic.12
Proportion of Land to Water: Influenced by the Apocrypha, he believed the Earth was composed of six parts land to one part water, reinforcing his belief that the sea crossing would be short.1