The old Wazoo tip of using the stadimeter is not as reliable. I use the Watch Officer's nearest contact function. When you click it immediately go to the nav map and mark your u-boats position with an x. What you do next is convert the bearing of the target (as given to you by the WO) into bearing due north by adding your course to the bearing of the target (subtract 360 if it is more than 360). Take that bearing and use the ruler with the compass option and click on your boat's marked position and line up the ruler's line with the bearing due north. Then you move it along this bearing for the range that the WO gave you. That is your first observation mark. All are done the same.
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