If your like me and Want to use Auto TDC, you have GOT to read this thread:
http://www.subsim.com/radioroom/showthread.php?t=165015
Forget lining up the numbered circles, that is old skool. Read the article at:
http://www.subsim.com/radioroom/showthread.php?t=165015
Read it a few times, then try it, I almost NEVER miss! Even at ranges over 3000 yards.