Caster is measured by attaching the contraption to the wheel and then turning the wheel outwards 15 degrees. By design, the end of the bubble level has a 15 cut to the fore and aft of the end of the level. So you just turn the wheel til it approximates that mark. Then you zero the right side of the bubble gauge. Once you zero it, you then turn the wheel inward to the other 15 degree mark and see what it reads.
Here it reads about 1/2 degree negative caster. (It was kind of hard to get the photo angle right so it looks like 1/4 in the photo).
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