CanuckBoat,
Welcome to the Subsim forums. It looks like you've done a good job at sussing out how to use the attack disc (Angriffsscheibe).
I don't use the one in the excellent OLC GUI because I built a home-made one from Hitman's templates (I like the added realism of using my hands instead of the mouse). It works similarly to the one in the OLC GUI, which is modeled after Joegrundman's U-jagd tools. I wrote a handbook (with Joegrundman's and Hitman's help) on how it can be used in the game. You can find it on my filefront page (see my signature), or look here...
http://www.subsim.com/radioroom/showthread.php?t=126824
By the way, Scale C, as you call it, represents a reciprocal bearing for the target. It is not actually a separate dial, but simply a second scale on Disc B. In the picture you attached, if your true bearing to the target is 320, then the target's true bearing to you is 140. It does come in handy.
One of my favorite ways to use the Disc is to determine a perpendicular attack course, then calculate the lead angle based on my torpedo speed and the target speed. Then I can fire a 0 degree gyro angle shot at any distance when the target crosses the lead angle. Hits almost every time, even from long range.
Keep asking questions. That's one of the ways we've learned what we have about these interesting tools.