You might also build yourself one of
Hitman's Kriegsmarine Whiz-Wheels, at least the backside of it that lets you do time/speed/distance calculations:
It's pretty simple to make: Print it out on cardstock, cut it out, and pin the three "wheels" together in the center. Here is where you can get it:
http://www.subsim.com/radioroom/showthread.php?t=114351
This is a set of instructions by
klh that shows you how to do many different calculations with this wheel:
http://www.subsim.com/radioroom/showthread.php?t=126824
Generally, I use the fixed wire method to determine speed if I don't already know it (ie., BdU tells me the convoy is moving at 7 knots). It's pretty accurate if you do it carefully.
With the fixed wire method, you have to be moving as slowly as possible. You set the vertical line right in front of the bow of a target ship. As soon as the bow hits the line, start the stop watch. Once the stern passes, you stop it. Since speed = distance/time, if you know the length of the ship you can calculate the speed. Also, it's AOB independent, except at very small/very large AOB angles.