I don't have any data to back it up, just my university-level knowledge of statistics. Although since you asked, I tried the calculator available online. It doesn't seem to work anymore, at least for me, even though it was updated to the new Ranked system. You can give it a try, maybe it will work on your computer: https://www.primedope.com/number-of-games-to-reach-legend-in-hearthstone/

One thing you can see however is that it takes over 500 games to reach Legend with a 55% winrate. Obviously, the further down you go from there, the worse it gets, and every percentage point lost is worst than the previous one.