The deep runners have nothing to do with the range setting. In Stock they most often run ~9ft deeper than set and TMO is none better
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So it is probably best to set the torps for a very shallow depth and rely more on the contact than the magnetic exploder. This is not to say to set the exploder to contact over magnetic, since the depth keeping problem might cause the torp to run beneath a ship with a fairly shallow draft.
Also hitting on a perfect 90° setup simulates a constructional problem, which results in tons of duds.