Arena Elo

Everyone starts at 1500. An Arena lobby is scored as a round robin: your duo's rating is the average of its two players, your expected result is your average win chance against every other duo, and your actual result is the share of duos you finished ahead of (1st = 100%, last = 0%). Both players gain or lose K × (actual − expected) with K = 64 — so winning an even lobby is about +32 and finishing last about −32, while beating stronger lobbies pays more. A player's first 10 games move 1.5× as fast and are marked provisional.