Regan's analysis works, in part, the way you suggest is better. He looks for tricky moves with non-obvious consequences and looks at one's success rate in those

Regan's analysis works, in part, the way you suggest is better. He looks for tricky moves with non-obvious consequences and looks at one's success rate in those. For the Niemann-Carlsen game, he identified two such moves and Niemann chose suboptimally for both of them. For one is those, Niemann played a nice that could have cost him a tempo. Definitely an inaccuracy.