Your distance of 600 yards seems fine to me, though I usually shoot the first fish at about 700 meters(~770 yards?) just to get enough time for a follow-up shot or evasion maneuver in case the first one doesn't do the job.
If he's heading straight at you, then the easiest thing to do is set the speed to 0, get a quick range reading and fire. If he's close to your 0 or 180, then range doesn't matter and you can just double click the "Send range to TDC" button, and it'll re-send whatever range was previously obtained plus the current bearing.
If he's heading at you but weaves, then dial in your best estimate of the speed and set the AOB to 10 degrees port(or starboard). Wait until he's finishing a weave and his AOB is about 10 degrees starboard(or port), get a quick range reading, re-send the AOB and fire. By now his bow should be pointing at you, and soon it'll reach the 10 degrees port you dialed in.
Shoot all torpedos at fast, contact(with late war torpedoes, I haven't tried this with the early war ones) and 1 meter, open tube doors before firing, and last but not least keep your thumbs up that the torpedo hits and actually functions
