This is the one I use:
http://myplace.frontier.com/~lee4pat241/id1.html.
I'm going to put it out there and say I don't sweat too much over the AOB. Particularly for merchants and other low-speed targets, I normally guess the ABO based on the difference between the apparent view of the target, and the profile view in the book. It doesn't make an enormous amount of difference at normal combat ranges.
So, if the target looks about half as wide in the scope as it does in the book, that's 45 degrees. Use that as a benchmark and you can get remarkably accurate shots with practice (only about 10-15 degrees out, which makes next to no difference up to about 2000m), with the added bonus that it takes about 2-3 seconds.
Real-life U-boat commanders generally did this too (since they could only have the scope up for a few seconds at a time), the AOB calculator wheels were more often used to get a course for a convoy, so you could plot its course and intercept later on.