I think lots of people use this method, but maybe without the manual maths.
The U-Jagd tools and the various implementations of them feature a marked stopwatch that has readings based on hull length (100, 150, 200, etc) and the RAOBF (range and AOB finder) tool that drops down around the periscope view. Most of the UIs that I've seen have some implementation of 1 of the above, if not both. Example:
http://www.subsim.com/radioroom/showthread.php?t=123373
I've found it to be the best way to go after getting the vessel properly identified or if you need to get a rapid solution ready.