Brows should begin directly above the middle of your nostrils. The highest point of the arch should connect the tip of the nose with the middle of the iris.
A great pair of tweezers should feel like they are vibrating as if they have a spring. If the tweezers don't do that, they won't work well at all.
If you have smaller eyes, you should choose a thinner brow that isn't very dark in color. This will open the eyes.
A thicker brow looks more feminine.
In grade school, my mother, who was a professional tailor, would make all my clothes. I became obsessed with designing them myself.
My goal is always to bring a sense of balance, symmetry, and proportion to each individual face. The brows are an essential starting point.