Using an ultra-high ELO chess engine to score each possible move, then reversing through the players moves and seeing how often it would have been a positive move (one that shifts the balance of the game in your favor) - or perfect move (not sure which). It is extremely rare to make 100% perfect moves in a game, let alone a series of games. Typical gameplay for high level chess player doesn't peak over 72-75% for a given series of N games. Niemann has several tournaments over this and several games with 100% perfect moves. The inconsistency is also a concern since he goes from mid-60's to 78/79 in a span of one tournament.

His games against Magnus were exceedingly high.