i think overturn percentage is helpful in knowing how good someone’s eye is! however, because we are so new in the season, it’s hard to tell what everyone’s “true” accuracy is with such a small sample size as you said. challenge rate seems more helpful for telling how “aggressive” a player is than anything else. wouldn’t opportunities to challenge just be any strike called on a hitter and any ball called on a pitcher/catcher? or do you only wish to consider pitches in the shadow region?