I think you should be more ahead of convoy, waiting right on his path with all stopped at periscope depth.
For that you need to shallow the convoy, estimate as close as possible his route, never approach below 10 Km, and full speed you go to the interception point you have estimate.
Best is to stand ready around 1000m of a convoy line. Right side or left side only matter the escort positions...
Sometimes they will detect you, sometimes not...so ready to launch or ready to dive deep and escape!
On side note it depends of what mega mod you are running.
While vanilla game remains easy, TMO or Ralles can be really tough
Good luck
