 So if a ship sailed 400 leagues on the "actual" SW course to Cape Verde, it would achieve an "avanzar" of 284 leagues (= 71 × 4) on the "intended" Westward course.Of course, the ship is not actually sailing SW to Cape Verde, but sailing W into the open sea.In other words, when the ship sets sail West from Seville, it knows it needs to sail 284 leagues on the West bearing before it reaches the implied Cape Verde meridian (point "C" on map), and should only start counting the 370 leagues to the Tordesillas line thereafter.