That’s true and that calculation was well known in 1492. That number would have made Columbus’ journey infeasible given the maritime technology of the day. However, Columbus either was unaware of that calculation or rejected it and proposed a smaller value, one that made the trip possible. This was the reason he had difficulty getting any support from European rulers for his voyage. Most knew that it wasn’t possible. Ferdinand and Isabella took a gamble because their main rival, Portugal, had gotten way ahead in establishing a sea route via Africa. It was only good fortune that there was a huge unknown continent blocking the western route.
Although Columbus grew up in Italy, his family were transplanted Polish nobles, with a tight family connection to Northern religious orders of Knights, who maintained a strong nautical tradition from the time of the Vikings.
He was reportedly familiar with Viking excursions to Vinland, and charts of the North Atlantic that extended West of Iceland. So he (and the Court of Ferdinand and Isabella) had lots of leads, but no current definite proof of what to expect.