Sorry Will I don't know for sure but you could work it out from simple trig. You know the length of the pivot and the height of a washer so you should be able to work out the tangent of the angle by dividing the height by the length.
SOHCAHTOA - gotta love secondary school maths from all those years ago - never figured that it would relate to the real world but hey, when you need to know your caster angle, you need to know
Hope this helps