I usually make my approach from one side of (perpendicular to) the convoy. Coming in from the side, I can present as small a profile as possible to the escorts. Coming straight in (0* relative) through the destroyer screen you will ALWAYS present your full profile to their sonar at some point, and usually to multiple escorts at "can't miss range" to boot.
If you are able to get around the convoy and set it up, try coming in from one side or the other. You should be able to close to within about 2500 yards (1000 yards distant from the escorts who may be 1500 yards from the nearest column = 2500 yards) without being detected. If you can dictate the timing of the attack, it is always better at night. Less chance of your torpedoes, or your scope being seen.
Taking your best shots at the most inviting, and still unwary targets from a range of 2500 yards, at a decent angle, is better than trying to get closer, only to be detected and try hip shots while in the middle of a bunch of stampeding merchantmen. Balance optimum range against against risk detection. Remember, if you are detected before your trap is sprung- all will be wasted and you will be lucky to nab any of the best targets and still get away to talk about it.
Number and placement of the escorts will dictate how close you can get, but you should be able to close the range without being detected to the point that you have a fair chance at inflicting serious damage and also escape without too much undue risk to your own boat.
Of course you "could" try the O'kane method and risk a surfaced torpedo attack. Running TMO though- I wouldn't advise it.
Crippling a ship will likely turn into a kill if you are able remain undetected after the fact... eventually the escorts will lose interest in you and abandon their search; and the wounded ships in order to rejoin their panicked flock... leaving the stragglers to your tender mercies.
