I guess the little diagram you ask about is indicating that SH3 uses a fixed wire method of measuring ship lenght. However IMO it is doing it very poorly and unreliably.
I estimate speeds using Hitman's excellent attack disk:
http://www.subsim.com/radioroom/show...=126824&page=5
Making an attack disk and learning to use it takes some effort, but it is definetely worth the trouble.
An easier way to do it would be to use the 3:15 rule or a nomogram. Look for Wazoo's targeting tutorial.
A third way of making an estimation would be counting the screws of the ships engine. Look for Raduz's hydrophone tutorial.
As for not having enough time to make the calculations, you can plan the shot beforehand, even set everything right manually in TDC screen (F6) and just wait for the right moment. While learning I would recommend you pausing the game when you feel like it. As a submarine commander you would not necessarily be calculating yourself, instead you could be giving bearings and watch stop times to the crew, who would do the dirty work. That said, a submarine commander would understand the calculations necessary, so it would be wise (and fun, IMO) to learn the skills eventually so well, that you won't need to pause the game.