Welcome to the boat mate.
Running metric changes all setting in the game to metric.
Sonar will give range in meters.
Don't pay too much attention to all the circles, they are only rough guides and not very good ones at that. You learn all these things by experience and usually by getting sunk.
Visual range depends on day or night, good weather or bad and also how good the lookout are. Generally speaking if you can see them they very soon will see you. In clear weather you don't want to get much closer than 7 to 10 miles.
Passive sonar also depends on weather, bad weather=bad passive sonar.
If you are attacking an escorted convoy as soon as you go to periscope depth to to silent running and low speed 3 knots or less.
Active sonar (pinging) If you hear it they will soon find you, go very slow, get very deep and silent running (you should have already been on Silent running) Active sonar has only one dead spot and that is directly behind the destroyer about 10° either side of their stern. Same with passive sonar. Active sonar range is about 1,700m' Passive about 2,500m but it may be more or less depending on weather and ability of the escort crew. You can't go under Active sonar so much but the deeper the better. The deeper you are the less chance they will find you it no guarantee.
The speed estimate thingy is all but useless. Get your speed using the 3 minute rule. Mark the target positing on the map (the end of the sonar line) and start the stop watch. Time 3 minutes and 15 seconds for metric (3 minutes for yards) and mark the new position. measure the distance and for every 100M equals 1 knot.
500M = 5 knots, 1000 M = 10 kn.
This will also give you a rough idea of the target course, do it again a little later and you will have a good course line and accurate speed.
Magic