The measure of an eye, of a thumb extended, of one cubit is seen in the result and how well things fit. We all know it when we see it. One can sense the art in craftsmanship by simply looking at the product. Some things we have made work better for the eye and hand. It is not luck. It is the experience of your efforts showing. It is the mistakes you have made along the way and their lessons learned that now inhabit your work.
Confidence is gained by your error and more importantly your understanding of it. What went wrong and why. We gain, in our designs as well as in our technique, valuable information that tells us what to do right this time, or right again.