Bhais, how do i measure the middle third?

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So, i got no issues with identifying the upper border of the middle third, but for the lower border im confused. My subnasale is pretty much too low compared to my nose, theres like a 3mm gap between the bottom of my nostril and subnasale, so it kinda makes it hard to pick one reference point. While measuring it, do i use the bottom line alongside the nostrils, or use the subnasale point?

 

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