To be honest as a nub What I did was drew a line in the general direction of the target's route then worked out how long it would take for the target (say at medium or ~8-9kt) to travel a certain distance ay 50km. Use the speed/distance charts in the F5 map screen for that. Then I work out how long it will take me to get to that point at standard (~12-14kt). If I can make it before the target then I plot a course for that point. If not I select a closer (to me) distance and work out if I can get there before the target. Once I find an intersect that I think will work I plot the course and go for it. When I reach the intersect or am approaching it I submerge for a sound check and see if I can hear the target (or anything else).
I never bothered to get too accurate as long as I was within a couple of game hours I considered it accurate enough to get close enough for a sound contact.
I also never bother with targets not heading the right way unless they were close. i.e; within my present grid square or in the next grid square.
Don't worry about being too accurate as if you have gotten your self ahead of the target then you can always head along the path he is taking until you make contact. Don't forget that they can change course (In GWX at least), so you won't always find them running on the same course as you mapped out originally which obviously makes them harder to find as it was in RL.
On longer contacts say four or five grid squares where there was a chance of getting an intersecting course I just used my eye and judgement to work out whether I'd be there and if it looked good went for it.
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