Altitude, speed (in similar a/c engagement not necessarily so in dissimilar) and angle on target are factors to consider prior to the merge. If there is a difference in angle to target between the two aircraft the one with the lesser angle is at advantage. I think Torquila said it well, you set course as close to your adversary as possible without colliding, both at a level attitude and same alt. Maneuvering commence after the two a/c pass by eachother. Any maneuvering before that basically tell you that the other guy is cheating because he is reducing the angle which he has to turn to get you in his sights.
It was meant as advice to those who are interested in dueling, because the merge is decisive. Which is why people fly the way they do in the MA, where you never see anyone fly like that. Usually you set up a lead or lag turn.