You need to get a general idea as soon as you can of the convoy's heading, and you can usually get this from surface visual observation. If you're heading in the general direction of the convoy when you sight it, I'd recommend cutting your speed right down so that you don't narrow the range to the convoy to bring you into its sight. This outght to give you a bit more time to figure out the convoy's general direction.
If at all possible, stay on the surface (gives you better speed and manoeuverability) and keep to max distance while you figure this out. Depending on conditions (i.e it would be different if you come into sight at short range and convoy is heading straight for you ... you may have to dive), you may even want to turn away from the convoy to show them your stern, that way you're better able to maintain or even extend your distance from the convoy while you get a handle on their heading and speed.
Now that you've bought yourself some time, start looking at the convoy (merchants, not the escorts) to get a handle on its general heading. As general as is it coming towards you, going away, and left or right relative to you (you may have already been able to get this) ... putting your UZO (is it TBT in SH4 ... the targeting binoculars) on a particular ship in the convoy will give you and idea of the convoy's left/right tendency relative to you.
With a general idea of convoy heading, you can now decide on your tactics ... run around and intercept from ahead, submerge for impending attach 'cause they're heading for you on a decent intercept. You can now make a rough course adjustment to bring yourself onto an initial track for run around or whatever, which you can then refine as you continue to observe the convoy.
Hope this helps a bit, I'm no whiz at it, but this works pretty well for me.
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