I'll usually shadow/end round at about 10-12km out at ahead standard without any problems, closer if weather permits. You have to watch your speed though-too fast and you are more likely to get spotted. If you feel the need to go to full or flank, get further out, maybe out of visual range completely.
You got to watch the escorts if you are doing this with a convoy though-they randomly patrol out from the convoy and then rejoin. It can be a pain at times.
Later in the war when the escorts are equipped with radar, you need to adapt your tactics and be real careful when shadowing and approaching. You may start asking yourself "Why do I want to attack this heavily escorted convoy anyways? They never did anything to me."