The question under every face-symmetry search is not really "am I asymmetric". Everyone is. The real question is "can anyone see it". That has been measured, and the numbers are kinder than the internet.
The detection thresholds
A systematic review of perception studies asked untrained observers to spot asymmetry in faces that had been altered by known amounts. The thresholds at which people started to notice were: eyelids about 2 mm, mouth corners about 3 mm, brows about 3.5 mm, nose tip about 4 mm, and the chin about 6 mm. On average, a lay observer needed roughly 3.4 mm of difference before registering it at all. Orthodontists, who look at faces for a living, needed about 2.5 mm.
Separately, in a study of 683 people, the smallest brow asymmetry patients could notice in themselves was 2 mm.
How asymmetric is normal
Very. In 100 consecutive patients measured before eyelid surgery, 93% had at least one eyelid or brow measurement that differed left to right by 1 mm or more. 75% differed by 2 mm or more. 37% by 3 mm or more. The average mid-brow height difference was 1.77 mm. Brow position is set by a tug of war between the muscle that lifts the forehead and the muscles that pull the brow down, and the lifting muscle is recruited unevenly in almost everyone.
Put the two studies together: most people carry 1 to 2 mm of brow or lid asymmetry, and 2 mm is right at the edge of what even the person themselves can detect. The stranger across the table sees nothing.
Why measure at all, then
Because the feeling of "something is off" does not come with a ruler. Someone with a 1.2 mm brow difference and someone with a 4 mm chin shift feel the same worry and search the same phrase. Only one of them has something a camera can see, and only the second one has anything to work on.
Symmi gives you the reading: brow height left and right, chin offset from the midline, eye openness, jaw angle on each side, 13 symmetry markers in total. You see the number next to the ideal band, and you see whether it is above or below the line where people notice. For most readings on most faces, the honest answer is "in range, and invisible to anyone but you". For the ones that are not, the app tells you which are structural and which respond to muscle, posture and fluid.
Sources
Wang T.T. et al., Aesthetic Surgery Journal 2017;37(4):375-385. Macdonald K.I. et al., Journal of Otolaryngology Head and Neck Surgery 2014;43:36.