From Dick O'Kane's method IIRC, I mark the ship's position, start the stopwatch and mark it again 3 minutes later.
Extending the line through the Xs gives the course line, and the ship's speed will be the distance in yards divided by 100... e.g.; in three minutes, if the ship travels 1200 yards, its speed is 12 knots. If it travels 900 yards, it will be 9 kt, or accurately enough for a close in shot.
I then move far enough ahead while remaining at least 5 miles from the target's course to avoid detection, then turn the corner to set up a submerged 90 degree position at 1000 yards from the course line.