Generally, the sea state and fog density will dictate how close I get in to loose my eels; always at high speed and hitting the frontmost rows of the convoy; at 90 deg. or thereabouts.
I have had good results (even against C2's) at firing ranges of 2000 - 2500 yds as long as my AOB is no less than 75%. If the escort is out of position I sometimes blast right in and penetrate the columns seeking out the tankers!
...particularly if there are slimy neutrals shielding the convoy and how determined I am at the moment.