Someone on the Torsk put together quite an impressive gadget (complete with carboard wheels that you could turn) to calculate it visually. I recreated what I could of it here (from the one VERY pixelated image I could find of it - hint hint AVG):
http://www.subsim.com/radioroom//sho...d.php?t=144202
Visual calculation of the value involved, all you need is an eye and a finger to use it (connected to the requisite brain of course). I find the less 'heavy lifting' I have to do, the less likely I am to introduce an error and the less busy the final moments of the attack.
This gadget was mounted above the TDC, so perhaps it would be good above the 3D TDC? Hmmm...