This might not be the answer you are looking for, but you cannot set manual range properly. In the stadimeter part of the TDC, you can drag the "mast height" plastic triangle clockwise to artificially drag the range out to about 1200 yards (I think). Also, once you've dragged the "mast height" plastic triangle, you can then use the stadimeter to "measure" mast height of an imaginary ship to see if you might be able to jiggle a fake reading into it

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If there are no warships about, you could use sonar to ping for range and send to TDC. But that's only accurate at 4000 yards or less.