Well, since I did the job I can't avoid the need of explaining it, even at the risk of looking as idiot
, so here it goes briefly (Note: Since I speak german I actually read the small white lettering in the centre, where it gives some incomplete instructions):
1.- Put the exterior wheel with the 180º mark UP (Not down, as you would think)
2.- Align central wheel (Truecourse) with your true course in the white triangle below the 180 mark of the outer wheel.
3.- Move the transparent cursor for Peilung to align with the actual bearing to target. If the target is aligned to receive a shot from your bow tubes, point the part with the "Vorh. Bug" there, if it is aligned to receive a shot from the stern tube, point the "Vorh. Heck" there
4.- Now turn the inner wheel so it indicates the AOB of the target towards your ship. To do that, align the observed or estimated AOB with the transparent cursor for "Peilung"
5.- You can now read off the true course of the target in the true course wheel.
Now, my guess is that the transparent slider with the +60/-60 degrees serves both to get intercept course and enemy speed, confirm the track angle in narrow shots, and also to calculate the spread for multiple shots (Salvos), but it's just a rough guess. Probably when I have built the tool I will be able to tell better.
In Puster Bill's picture, the set-up would be as follows:
1.- Our U-Boot true course is 273
2.- The target has a bearing of either 118 degrees to our starbord side or 297 to our backbord side, depending on which side of the pointer you chose to use (For bow or stern tubes shot)
3.- The target's estimated AOB is either 88 port or 92 starbord, depending on the above
4.- The target's true course is as a result 113
Now, placing the the transparent slider with the +60/-60 degrees aligned in the middle between target's course and our bearing to the target would tell us the exact intercept course to the colision point between us. So changing our course to that new true course would bring us to colide with him (If keeping both UBoat and target had the same speed). This probably is used to start trying a match of speeds and thus get the target's speed via the backside of the ruler while heading to intercept it.