I'd put it at 450 yards to arm.If going against a DD and going for a down the throat shot, I fire at 600 yards, usually they cant evade them that close but have fired at 500 and fish armed.
Your Mark 10's were armed, they were duds.If they do not arm, they just hit and crew makes no report, if they report torpedo as a dud, it means that it armed, hit but failed to explode.Mark 10's rarely dud but it does happen from time to time.
