Alright, so I have a contact and I put an X on the navmap with the pencil tool. I make this first mark at 7:20. I continue on my approach and at 8:14 I make a second mark on the navmap. Usually I'll have a small notepad next to my keyboard and I'll just jot down the X marker number and the corresponding time for each like so:
1 720
2 814
Now I pull up the navmap screen in MoBo. I put a node on mark one, hover over it and press T to enter time as 720 for the node. I plot another node on mark 2, hover over it and press T to enter time as 814.
Now I right-click on the 814 node and choose Connect. I select the 720 node to connect to. Now a line connects the 720 and 814 node.
I hover over the 814 node and press the Y button (TSD tool) and the elapsed time of 54 minutes is displayed on the line along with the target speed of 4.5 knots.
Now I line the target up in my sights, set tubes 1 and 4 for a 2° spread shot and...
Two solid hits! Looks like my speed calculation was correct.
As you can see, the 3 minute rule doesn't really mean anything to MoBo. In this case I happened to allow 54 minutes to elapse between the two observation marks. If you happen to get some crazy result for speed showing up on the line that just means you forgot to adjust the MoBo map scale (notice my MoBo map scale is set for 8.2km to 662 pixels).
...and now I'm off to go turkey huntin, I'll check back in to see how you're doing when I return Wednesday. In the meantime feel free to post questions; there are some real MoBo experts lurking about these forums.