Well
For one thing, you should zoom in your map as much as possible, and still have the two ships on screen. That will ensure your ruler tool gets started and ends on the center of each unit icon.
The periscope view has marks for measuring range based on the mast height as indicated in the ship manual. I don't use that method myself.
I believe the stock game, for sure mods, have a command for the sonar guy to give his estimate of range, or send range to TDC. Among the three results you get, pick your best guess or an average.
This is not an exactly perfect game, but even real life calculations would be subject to error.
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