Use the new creature’s power and toughness at the time the ability resolves to determine the base power and toughness of Eldrazi Mimic. If that creature is no longer on the battlefield at that time, use its power and toughness when it left the battlefield. (Keep in mind that those values may be negative, for example if a spell like Spatial Contortion giving it +3/-3 is what caused it to leave the battlefield.)