Hard to say because the U.S. never adequately tested the magnetic detonators either before or during the war so there is no actual hard data. We just have to rely of patrol reports.
We know that at the beginning of the war skippers were setting the torpedoes to run 10-20 feet below the keel, none exploded. Of course, torpedoes also had a depth keeping problem which could cause them to run 6-10 feet deeper than set, so all these torps might have been running 16+ feet under the keel.
6 feet/2 meters, which is what I think both RFB and TMO use, is probably the best estimate of the reliable maximum range.
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