You can do this from the Attack periscope and/or the Torpedo Attack map screen.
From the attack periscope, you have those two panels on either side of the view window that give you the torpedo settings. On the right panel there is a purple light, you can click that to lock and unlock the torpedo setting dials.
If the light is on, then when you swivel the scope, you will see various dials spinning, and you will not be able to adjust them. Click the light off, this decouples the scope from the TDC and allows you to change the dials.
From the Torp Attack map screen, there is a button near the bottom row of 4 dials you can see, if you click the button it locks (red) the dials, or unlocks (green) those 4 dials for manual input. Its the same function as the purple button in the attack scope view.
To set up your shot, decouple the TDC. I'm going by memory here so I'm gonna use the Attack Map screen. The dial on the furthest left of those 4 bottom dials should be set so it points straight up. This is equivalent of turning your attack scope to 0*.
Then I like to set the next dial to say 1000m range, tho this is arbitrary really, I just like the torpedo path line to extend past the sub bearing graphic.
Dial up the correct speed the target is moving at, and set your torpedo pistol and depth if you haven't already.
Then adjust your AOB to 90* in the proper direction, and you will see the torpedo path from your sub (and the torpedo gyro dial) will offset. This is the direction the torpedo will travel when you fire it. As you said, it will usually deflect 20* or whatever to one side or the other depending on the AOB you dialed in.
At this point, you can lock in the TDC, and whatever direction you turn the attack scope, the TDC will update the torpedo gyro, and the AOB for the shot. This is great for in a convoy, as you can swivel the scope around (keeping in mind the max torpedo gyro limits) and fire your torps at will (careful if you use impact only settings). This is where dialing in a more correct range for the target is handy, as the more off 90* you shoot, the less accurate your shots will be using this method, because the torpedo goes straight out of the sub for a preset distance before it begins its turn.
Alternatively, you can look at the calculated gyro angle for the torpedo, and adjust the first dial again to match it in the other direction, so that the torpedo will go straight out of the tube. Since you use a 90* AOB setting when the target is directly in front of you, this shot is very high probability of a good hit. I leave the scope decoupled so I can look around, but I'm aware of the calculated gyro, so I will point my scope in this direction, and when the part of the ship I want to hit passes the scope, I fire off the torpedo. I'm not sure of the inherent advantages to leaving it decoupled, tho I will use this way for hydrophone-only shots where I don't raise the scope at all to fire, so I don't care where the scope is pointing anyways.
I think I got all the steps in there, doing this from memory at work...
