Try the single mission Happy Times: you will encounter a convoy close-by which is almost devoid of escorts....the are rushing in from a distance though. You can even pick a U-boat of your liking.
That gives you just enough time to play with the maths a little and pick of some nice merchants....if you approach it well, you can easily achieve a perfect 90 degrees firing position; the mission is not called Happy times for nothing!
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Daddy cool 
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