I guess you have to accept the fact that when you raise the difficulty it makes things more complicated. Since I also use manual navigation I miss also more shots, but in a way that's what I have been looking for. If you hit 100% of the time, it gets a little boring after a while...
I still use the 3'15" method and work it the other way around. Once I have plotted the target estimated course, the reverse bearing and distance give me the estimated position of my sub and I plot my own course from there.
The point is that all these are now estimated positions, and if your estimation is off the mark, you're in for a miss. But that's what they had to work with at the time...