Wait for the enemy to approach that point. It may take several plotted waypoints to have the U-boat lined up straight with the perpendicular line. I find a location that is about 400 meters along the perpendicular line and plot a course to reach it. With the ruler, I draw another line perpendicular (90 degrees) to the predicted course at a position I believe I can reach with my U-boat before the enemy gets there. Using the ruler tool, I draw a line across the two marks to predict the ship’s course.
I wait for a while (exactly how long depends, but perhaps 5-15 minutes of game time at 1x) and raise periscope and mark the ship’s location again. Go to periscope depth (if not already) and keep periscope down.
After spotting an enemy ship, I mark its location on the map.
