This is probably the main reason why I try to make short shots whenever posible (500 - 800 m) because it is more forgiving. If I'm a little off on the distance I will hit either more in front or more in back than my aiming point, but I will usually hit somewhere. The longer the distance, the more chances to miss completely if you're off in your estimate.
Hey but that's why they didn't come back from every patrol with 100.000 tons on the log...
I now feel quite happy when I get home with anything above 30.000 tons after a 4 weeks patrol. (not talking after the "happy times" when I am already very happy to get back home
)