Manual targeting is nice indeed.
A good way to know the target speed. Is when you know the ships length.
For example. The ship you want to shoot at, has a length of 125m.
Make sure you are matched perfectly with the target ship. So you see the entire side of the ship. (Angle On Bow 90)
Something like this
Make sure your own boat is not moving. Point your periscope line in front of the ship. Start a stopwatch timer. Wait for the target ship to pass your periscope line completely. (Front to aft)
When the aft passed your periscope line. Stop your timer. and see how much you got. Lets say you got 43 seconds, then you calculate the formula.
You do the formula. The ships length (125) divided by the timer (43 sec) and multiply by 1,944. That would give you around 5.6 knots.
The formula is ( D / T ) * 1.944 = S
You got different ways to know the target speed. But i always use this haha. Its not bad to do a couple of speed checks. To be sure
The periscope needs to be kinda stable. Its a bit tricky to use the periscope line. if you have high waves or during a storm, when your boat is bouncing up and down
You can also use the UZO on the bridge. Attack periscope or Observation scope also work. You just need that vertical line.