Cain,
I would suggest you try and find a set of round feeler gauges at an auto parts store that are normally used to measure the spark plug gap on cars etc.
From memory (not been a auto mechanic for awhile now!) there should be the right diameter gauges on something like this.
I wil post a picture of one if required, would just have to find it on the net!
Pretty sure snap on used to make a set...
Hope that helps. It would be alot easier and far more accurate measuring it this way rather than with calipers, particularly at that size.