Solo, try using this
"High Realism Tutorial" with the Calculator.
The design you downloaded is the same as mine, just better graphics. You are correct to figure the outer most dial on the calculator represents the "relative bearing" either of the heading of your sub, or the view bearing(found along the top of the periscope or TBT stations). The inner dial represents the true north compass bearing (as it relates to the map). The smaller, top most dial represents the target ships heading and/or Angle on Bow.
Although the American side has the Position Keeper dials to relate very similarly to the hand held Calculator; if you play the German side, the Calculator is very useful.
Try reading the tutorial, or even use the following target ship mission to practice in an environment that will help you understand map plotting and ship target AoB recognition. Here's the link to the same
"Target Mission" as used in the tutorial.