If you view the chassis from the top you'll see that the ball joints give you a dimension, shown here as X, this is the castor when viewed from the top.
This dimension doesn't change when you fit the inserts (either type) what happens is you move the upright around. It's the line drawn between the ball joints that determines castor, the centre line through the hub will change, but has no effect on the castor.
Kind regards
John