Between %52-53 actually and people reach rank 1 with %46 or something winrate. My MMR is higher than that though.
Besides win/loss doesn't matter as much as you might think because some games give 40 points, some make you lose 110 and so on, which means MMR loss or win doesn't equal win/loss. Also I didn't abuse party system to get ranks %70 of the games were solo so I feel satisfied.
Besides, matchmaking is complete garbage. With some luck, a player who deserves rank 30 will easily rank 1 just by abusing the broken system, and I've seen it happen. Most games at rank 1 are decided by whichever team gets that rank 48 guy who won his first 3 games and the game thinks he should be at the same level with rank 1 players with 400 games.