I preface this by saying that as yet I have not worked on the front suspension of a Mk2 and my understanding of suspension and steering geometry is far from perfect, so if I’m wrong about any of this please let me know.
From the look of the diagrams, castor is set by moving shims between the front and the rear of the upper ball joint. I assume that there is also a fixed amount of castor provided by the swivel inclination although I don’t quite understand the relationship between this and measured castor.
I assume that the upper wishbone halves are in a fixed position relative to the fulcrum shaft, i.e. you can’t move them fore or aft like the castor adjustment on an E type. If so there is a limit to the amount you can move the ball joint back (or forward).
So the maximum positive castor you can generate is when the upper ball joint is right back against the rear half of the upper wishbone with all the shims and packing piece at the front.
I recently had some argument with our local government vehicle inspectors over an E type I was trying to road register. They claimed that the steering didn’t adequately self centre (it drove exactly the same as my own E types and the dozen or so I’ve been involved in restoring).
Eventually I dialled in as much positive castor as I could; about 2.5 degrees (spec is 1 to 2 degrees). This did make the car centre better and I got the car passed but it made the steering unpleasantly heavy at slow speeds. As soon as I had the plates I wound it back to 1 degree.