I think you need to have the ship locked (Even if not identified ) for the stadimeter to work. See this example:
I moved manually the transparent triangle with the red mark (Which indicates estimated mast or whatever heigth) to the "50" feet mark, as that is my estimate of heigth for the funnel of a large cargo like the one I see here. Then, it is just a matter of using the stadimeter to align waterline with funnel, and there you go...opposite the red mark of the triangle you can see the resulting range: Nearly 11000 yards

(I have the horizon set at 20000 yards)