I usually end up assuming a mast height of anywhere from 30 to 35 m for most merchants, and then use the centiradians in the periscope to get a feel for range. Just mentally divide target height by the number of marks he takes up to get the rough range.
So say you estimate target height as 30 m, and at 1.5xmagnification he takes up three marks, he is 1000 m away. If he takes up three marks at 6x magnification, he is 4000 m away. For 6x magnification you need to multiply by four.
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