To gauge by hand and eye is perhaps the pinnacle of craftsmanship. To reach a level of ability where you no longer have to measure everything with a rule, but can by your own good judgement achieve a sense of proportion and balance of harmony of the thing that you are crafting.
This is no easy feat. This is something that takes years of patience and skill and ability, and most of that ability is between the eye and the brain, and the soul. To know when something is right without having to check it.
It is a thing of feeling, of sensing, as much as seeing, skills that are hard to teach and even harder to acquire. They are mastery of a craft.
A peculiar thing begins to occur if and when you can reach this point. You can begin to apply it to other crafts, often without much knowledge of them, because it is not the hand work, but the eye work, and the soul work. It is the feeling of a thing in place, the hands will follow.
And it all comes from a place of knowing, knowing on a soul level when a thing is right. A thing you can feel right inside of you. This is not something you can understand from a book or video or podcast, it’s not even something you can acquire from a teacher, but it is something you can find if you look for it, and the place it resides is inside.
This is the spirit level of the soul, and the only way to it is through practice, a lot of practice.