Having just started manual targetting myself, I find the "Fast-90" method the easiest to use. The German TDC will automatically calculate the AOB for you. I plot the contacts course, and extend it. I also start timing the contact which gives me its speed (3 minute rule). I then park myself perpendicular off the estimated course of the contact, I try to get within 2000m, and preferably within 1000m, but I've been successful at much greater distances. Once your sub has stopped moving its easy to set your attack periscope to zero degrees, since you know the targets estimated path its very easy to get the range, and since you've been timing the contact you can get it's speed. After you plug these two variables in, no matter what direction you point your scope the TDC knows the AOB. I don't lock the target. Once the target passes the periscope wait till the part you want to hit passes and fire off your torpedos. If you want to fire more than one, move your scope and shoot again. Only thing is dont forget to open your tubes, this seems to take about 10 secs and is enough to throw your aim off. Note if you fire too far off of a zero gyroangle it will mess up your range setting a little, but I only find this on the long shots, generally for this method the range doesnt even really matter as long as your within a few hundred yards range.
This also works quite well in the US boats too. There is basically no math required for this method, and you can very quickly target numerous vessels in quick sequence. I found the stadimeter useless to me, and I'm terrible at guessing AOB, and by the time I'd calculate it and input it to the TDC it would have changed enough to make my shot inaccurate. So I use the above method which for me is very simple to use. Your mileage may vary
