Bearing at which you should fire at 90 angle depends from distance and speed of target. Once this is given to the TDC, move your scope ahead of 0° towards which side target is coming from until he gyro angle is 000. Should be somewhere in the 10 to 20 degrees lead for average distance between 500 and 1000m. Anything worth shooting at I usually send a 2 torps salvo with a small spread like 2 or 3 degrees. And I try not to attack a convoy by daylight (learned that by loosing a few subs...
) Don't forget to open tubes before...