And Aaronblood's MoBo program is performing a perfect bearing rate analysis as done on a WWII sub's plotting table and in the above referenced manual. It mathematically does what you do with your analog ruler.
MoBo is a free download from right here at Subsim. I gave you a link to the whole forum dedicated to MoBo. You can easily find the download link and lots of help in using Aaronblood's indispensible program there.
Hey Aaronblood! You gotta learn to toot your own horn! I'm happy to help, but I can't be everywhere MoBo needs to be promoted.
MoBo: it's what's for dinner.
OOOOOOOOO! This is a relative course you're plotting with the bearing rate plot on the maneuvering board. Don't forget to convert relative course to true course before you start filling the water with torpedoes!