I suggest using the 13 minute rule, and divide by 4. 3m15s, 6m30s, 9m45s, 13m0s.
The stopwatch slower hand doesn't go beyond 12 minutes, and then just one run around from the seconds-hand. You'll need that quarter knot preciscion for shots a bit further out. Well, when you're up for it, ofcourse.
It also teaches you to be patient.
Also, with the CTRL button in combination with the up/down buttons for the respective periscopes they go all the way automatically. So you do not need to wait until they go below the surface to start plotting.