[mookiemookie]

The PK targets the "locked on" portion of the ship. To aim for certain parts of it, use the degree offset dial. For instance, if the periscope locks on the center stack of the ship, and you want to put a shot right under the bridge, unlock your periscope, move it over to where you want your torpedo to hit and note the change in bearing from the original "locked on" position. The degree change is the offset you use.

As joea mentioned, this is the way the TDC worked in real life.