According to this video,
C0/C1 flexion is 10°, extension 17°
C0/C7 flexion is 64°, extension 63°
Which means 27/127 ~= 21% of the whole neck flexion/extension happens at the C0/C1 level.
Might be accurate, though other sources mention up to 50% (https://www.physio-pedia.com/Cervical_Vertebrae).
Other sources mention these numbers:
The cervical spine’s range of motion is approximately 80° to 90° of flexion, 70° of extension, 20° to 45° of lateral flexion
Normal flexion to hyperextension at the atlanto-occipital [C0/C1] joint ranges from approximately 15° to 20°
Normal cervical flexion is usually approximately 80º. […] Normal cervical extension is usually 50°. Total range of cervical motion from full flexion to full extension should be 130°
Cervical Lateral Flexion […] Normal flexion from starting point on either side is 45° and the total angle of maximal lateral head flexion should be 90°
https://www.ajronline.org/doi/pdf/10.2214/ajr.130.2.317
(offline copy: ajr.130.2.317.penning-1978-normal-movements-of-the-cervical-spine.pdf (1.3 MB) )
Now, this is getting interesting. It seems C1/C2 also may have significant flexion/extension range.
I will leave it here.
Also, this in particular demonstrates the (approximated) C0/C1 motion
The whole neck flexion/extension
