Brian, fingers is all I've ever used for measuring thickness differences. I never learned to use calipers and wouldn't if I knew how. I want to make my "primitive" bows as simply as possibly yet still be effective. It is amazing how sensitive out touch is. A friend of mine that grew up in Cali had a physics teacher that did an experiment with the students to measure that sensitivity and found out that we can feel a 10,000th difference with our fingers.
Nice bow, BTW.
