The U.S.N. did things a bit differently than the Kriegsmarine. One was they didn't salvo thair torpedoes. One reason for this is at the start of the war they had massive shortages of torpedoes so the rule was one to two torpedoes per ship (from silent victory by Clay Blair JR.).
I use the angle setting to offset where my torpedoes will hit an enemy ship. If you leave it at zero it'll hit on or near where your crosshairs are pointing (depends how accurate your solution is). Each mark on the dial represents one degree, these corespond with the periscope marks:
Small marks = 1 deg in low power
Large marks = 1 deg in high power (with the small marks showing 1/4 degrees).
So you can aim your torpedoes with more accuracy eg hit below the funnel to get the boilers and engine room or hit below the turrets of a BB to try and knock those out.
To sink a fleet carrier would probably take more than two torpedoes unless you got very lucky and hit the magazine or fuel storage areas.
Also the magnetic detonators used for below the keel shots were removed by 1943 as they simply didn't work.
Hope iv'e helped you out

good hunting