If you are using a mod that causes the map contacts to not have their directional 'tails' then this doesn't work. Otherwise it's really this simple. Put away all the tools and calculators.
Man I can't stand that I can't paste on this forum. How can I do it?
Here you see I get a contact. I take the course thing and make an intercept. The 'target leg' is drawn straight through the target's directional tail, that establishes the target's course. The 'intercept leg' is then drawn to intersect the target's course at a distance that assures we arrive ahead of the target. Rarely will any merchant be making more than 14 kts. So all you need to do is make each leg of the intercept roughly equal, or a little longer for the target leg, then go faster than the target. You'll arrive ahead every time (unless the target changes course, unlikely).
I may be wrong, but it seems in reading your posts that you desire to intercept the target at a right angle everytime. I say you should get to the target's course early, then travel down the reciprocal of the target's course and meet them head-on.