Here's a photo of the real thing:
http://www.subsim.com/radioroom/show...&postcount=116
In his thread Hitman provides the downloads and instructions to make a working model. I prefer the in-game one, but a lot of folks like the hands-on experience.
http://www.subsim.com/radioroom/showthread.php?t=114351
The rest of the periscope targetting tools are also accurate to the best of my knowledge. I personally prefer the simpler U-jagd Tools mod, mainly because it let's me keep using the 6 Dials-Simfeeling mod, which in turn gives a clean, uncluttered screen.
http://www.subsim.com/radioroom/showthread.php?t=123373
http://www.subsim.com/radioroom/show...ght=Simfeeling