Hi Maz,
I copied my other post and changed a few words.
If it was me, I would take the springs out of the wishbone and reassemble,
hold a steering rod at a position that a rack ball joint length you have in mind would be at driving straight,
then use a jack to raise and lower the wheel to their full limits,
to see the forward wheel alignment changes.
That will give you the changes to work with, also One(1) degree of wheel movement is acceptable.
This way you will see what height the rack changes will do per ball centre changes.
As said earlier, Holden would of spent Millions getting the 515mm ball and rack centre to centre right.