I said "triangulation" earlier....but let me be more accurate....it's actually about keeping parallelograms.
You want the tierod link and the control arm to maintain a perfect parallelogram. Both in the horizontal and vertical planes......No matter how you turn or how the suspension travels.
A parallelogram with sides of a given length can have any range of angles between 1 and 179. But this parallelogram must have 2 sets of EQUAL sides or else it's not a parralellogram.
this means some distances must be equal.
rack end to end = A-arm pivot width.
Rack postion above a-arm pivot = height from lower balljoint to outer tierod end@knuckle
Rack position forward of the axle centerline = horizontal distance axle centerline to outer tierod@knuckle
Any of these being off will make trapezoids rather than paralellograms as things move. The angles of a trapezoid cannot be changed without affecting the length (distance) of the sides.
This is why the toe of the wheels relative to each other will change when you use a rack wider than the pivots.