It hasn't been revealed yet but if I had to guess, I would say probably around 400 because the Level cap is 40. Because when Vanilla Destiny came out, the Level cap was 30, and the max attack value was 300. When the Dark Below dropped, the Level cap was 32 and the max attack value was 331. When the House of Wolves dropped, the Level cap was 34 and the max attack value was 365. So it would only make sense for the max value to be like 415 or so because the new Level cap is 40.