Many Classical Guitar Students just let the left hand thumb fall anywhere where it feels comfortable.
The result ? An inability to play even simple chords and fingers not being able to reach fret locations.
So keep the thumb slightly closer to the hand from where the exact center of the back of the neck is located and also positioned directly in between the second and third fingers.
This will balance the left hand and also allow it to be ready for any fingering patterns.
The Left Hand Thumb
on the Back of the Neck