From what I saw when I did my "mirror experiments" last summer, the positioning of those additional lnbfs is little more than trial and error. Ie, there is only one focal point of a dish like that, so when you put on a second or third lnbf, it will not be in focus, so I think it's just a situation of being able to move it around to find a spot that isn't too bad. That mirror experiment was really interesting, as the reflections from different parts of the dish moved in all sorts of different directions, like ants swarming around looking for food.
Since people seem to have acceptable results doing this sort of thing, there obviously must be some good spots that work well, so I would think that it would be best to hand hold the lnbf while looking for the ideal spot, then adjust the bracket to put the lnbf in that spot. In a bit more than a week, we'll be in one of those sun outage periods, so it would be a good time to try the mirror method to locate the best position for the lnbf. Ie, you can determine the time when the sun is behind the sat you want to view, then find the spot where the mirrors focus best.
http://eskerridge.com/bj/FC90-sg2100.html