You mean a method for finding the convoy or a method to find it's exact course once you've spotted the ships? Well here's something for both (probably someone has better methods but here are mine anyway)
To find the convoy you should chart a "probability corridor" or whatever it's called, just like in the navigation tutorial video, but make it get wider and wider as it gets further away from the convoy's position. A little math can ensure that your progress along the corrider is quicker than theirs (if you're behind them) or slower (if you're ahead of them). This works well for me.
To get their exact course... one method is simply approach the convoy and get into line ahead of (or behind) one of the convoy lines... aim down the scope so you have a nice neat line of ships sailing away from you, and hey presto, you've got their course
. Retreat, flank, submerge... and attack
assuming they didn't change course
