If the target is still far away you could also use this tool with periscope bearings recorded (and the differences between them calculated) to determine what the AOB was at the 1st bearing. Then calculate back that many degrees to the bearing where AOB would appear to be 0. Then 180 degrees from that would be the bearing indicating the target (relative) course. By adding your own course and reducing any multiples of 360 degrees you get the true course of the target.
[edit] It is important that your boat is not moving or turning while doing these bearing measurements at fixed intervals.
http://www.subsim.com/radioroom/showthread.php?t=147719