The thing to consider here is that the Sierra upright is being used, this is designed for a Macpherson strut, so when used in our application provides a few "issues"
The picture below shows why, the diagram on the left is the hub using a concentic (in line) insert, the right with eccentric (offset) insert. Both have the upper and lower pivot points in the same place, the only difference is the insert used.
As you can see, with a concentric insert the centre line of the stub axle is forward of the line drawn between the pivot points (castor angle), moving it above the the lower pivot. This reduces the trail, this is what gives self centering. By using an eccentric insert you can see that this rotates the hub backwards around the lower pivot to get the stub axle back in line with the line drawn between the pivot points, therefore increasing trail and improving self centering. Castor is only increased by moving the upper pivot point backwards, either by moving the whole upper wishbone or by making a new one.
Hopefully all that make sense.
Kind regards
John.