Fastfed (and anyone else), since you're attempting to learn manual targeting and trying to map plot the target, I'd suggest you download and print out this reference
"High Realism Tutorial". It's designed to have a new player become familiar with the Position Keeper, Attack Data Tool, the several tools used on the Navigation Map, and the process in getting Speed, Angle on Bow and Range/Target Relative Bearing to work for you in acquiring a firing solution.
At the above thread you'll find my AoB Calculator which is a handy handheld device to figure target AoB, or it's tract heading. The Position Keeper does the same duty, so you don't have to have it, but it is helpful when at the Nav Map. Also, there's a "High Realism Mission" which can be added to the game with JSGME so you can follow the same target that's described in the tutorial. If this single mission isn't enough, download this mission
"AoB Test", it has several targets heading towards your submerged sub at different angles/speeds for practice too. I made it for practicing with OTC, but it'll work with just about any mod. Once the practice missions are activated through JSGME, you'll find them in the "Single Missions" menu at the "main" menu screen.
One thing about your example images of the Nav Map......Why aren't you using the Nomograph? That's the three vertical lines thingy on the right side of the map! If you know the distance a target traveled, and the amount of time it took to travel that distance.......drawing a straight line connecting the two figures will point you to the speed the target is making. Simple as that.