There is a really nice convoy interception tutorial
here.
The ghosted image is flat out wrong in many instances. It assumes certain conditions like some arbitrary speed you may be going, but doesn't update for different circumstances. I would call it bugged. If you follow that tutorial, you can get reliably where you want to be.
On that note, it would be nice if we could actually get a time stamp on the random ship sightings like in SH IV. Otherwise you have to pretty much see the contact appear on the map to get accurate intel.