I use different tacticks for calculate speed.
you will need a small exp. in mathematic
Thise how i do it
first you need to know the leight of the ship.
Lets take a medium cargo for example - her leight is 140.5 meters
now use our vertical line in uzo our periscop view.
Symply start to count in for how much time the ship will pass thise line without moving it (i` am sorry English is not my mother language and its very difficult for me to explaine properly).
Now
Lets the matematic begin
Lets say that the medium cargo pass the line in 55 sec
now we have 2 infos
140.5 (the leight of the ship)
and 55 sec
now for the fune part
140.5 to divide 55 = aprox 2.554545 ok - thise is miters in sec
Now lets convert thise in knots
2.554545 To increase
1.946 = 4.97 knots
Thise is the speed of your target. Sorry again for my very pure English but if some one found my metod interesting i can make a small tutorial with screenshots
