If the castor is too steep (too vertical or past that) the steering won't self centre and at speed becomes (dangerously) unstable. If the castor is too great, the steering becomes too stable, heavy and less responsive.
The angle is relative to the road surface, so lowering the front or back relative to the other end can affect the castor quite a bit. That's why you should have the car in average trim, usual level of fuel and an average person weight in the drivers seat etc.
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