Yes, it's really easy to forget that all we're doing is drawing a scale model of the firing solution and start to confuse 4400 yards on the graphic solution with an actual 4600 yards distance required to make the shot.
Keep practicing and it all becomes automatic. But when you're first learning it's easy to become confused about all the details. Keep up the good work!
Hitman, the fact that the Germans did it could have lent a halo effect to the assumption in American sub movies that Americans did it too. The Germans had some important advantages. First the Atlantic was crawling with hundreds of pretty identical cookie-cutter Liberty boats. Once you learned the RPM/speed curve for one you had the keys to the city. Also the open nature of American and British societies, where information valuable to enemy combatants easily enters public knowledge, made information on our merchant shipping much more accessible than the relatively closed society of Japan.