On deciding on how to arch your eyebrows one of the most important part of designing the brow, we are going to use other features for a reference. For our model here, I've decided on a small arch for her eyebrow shape because she doesn't have a lot of eyelid, and she has straight brows.
So placing a ruler (or something else straight like an eye liner) on the tip of her nose as a reference, and taking the ruler over her pupil brings me to a section of her brow that you see above. This is going to be the "peak" or highest point of her arch.
Another reference point is the top of the nostril, still aiming for over the middle of the pupil for a medium arch, but the far side of the pupil for a large arch. (Tip: right nostril for right eye, left nostril for left eye).