to understand caster, look at the front end of a motorcycle, the triple tree is at a forward angle, meaning if 90 is dead straight upwards, the angle of the triple forks intersects the vertical and places itself ahead of the perpendicular.

This provides the stability, the more the angle the higher the caster. To measure it you need a 90 degree point of reference of the spindle (or ball joint), ideally done with a jig that mounts to a solid area of the balljoint housing, e.g. the W124 FSM describes a tool that hangs from the ball joint clamping bolt, the little divot on the bolt ends is provided just for that. It has a string that goes across a scale to indicate caster degrees

In cars where the joint is hidden or unreachable reliably then you can measure with the camber difference.

If you have some slop somewhere in the spindle axis like worn arm bushings, worn ball joints or something that can throw it off - your measurement will be cuckoo.

__________________
2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)