As we shall see, the likes of Columbus and Magellan employed a much-reduced estimate of the Earth's circumference.
They had to. The voyages they proposed were well beyond the capabilities of current maritime technology.
In addition, Marco Polo had located Ci[pangu (Japan) around two thousand kilometres east of Kublai Khan's empire. Based on that figure, he could have been in the vicinity of his objective.
And if he was not, he could have been approaching an equally desirable objective. Descriptions in The Travels of Java the Great and a string of fabulous locations that lay beyond it would be enough to pique the interest of any would-be adventurer.
[Great Java] is the largest island in the world, having a compass of quite 3000 miles . . . [and] is immensely rich. They have pepper, nutmegs, spikenard, galingale, cubebs, cloves, in a word, all the precious spices one can think of. Great numbers of ships go thither, with many traders who buy sundry wares, from which they obtain much profit and gain . . . The merchants of Zaitun (fn) have in the past obtained great profit from the trade with this island and still do so. The great part of the spices sold in the world come from this island.
(fn: "There seem to be no good reason for doubting that Polo is here referring to the old kingdom of Champa extending up the eastern coastline of Indo-China." (Colin Jack-Hinton, Marco Polo in South-East Asia: A Preliminary Essay in Reconstruction, p. 48, n. 2.)
(fn: "Quanzhou was China's major port for foreign traders, who knew it as Zaiton, during the 11th through 14th centuries. It was visited by both Marco Polo and Ibn Battuta." (Wikipedia)
Following the directions given in The Travels from Champa (the southern end of Vietnam) would land a traveller very close to modern-day Java. Following Marco's directions for another two thousand kilometres in search of "a continental province, called Locac which is very large and rich" would land the traveller in Australia. However, the destination hardly matches the one the Venetian describes:
. . . In this country, there grow immense quantities of brazil-wood and ebony. They have a great abundance of gold, so great, indeed, that no one could believe it without seeing it. They have elephants and game, both beasts and birds, in great plenty. From this region come all the porcelain shells that are used as money in different countries.
One might think there is no possible connection between continental Australia and Christopher Columbus. However, according to his mistaken geography, he thought he was somewhere relatively close to the modern-day Philippines or Indonesia.