AILA/CBP Liaison Committee offers a practice pointer on determining the appropriate period for admission on a TN for a Mexican citizen.
https://atomic-temporary-10880024.wpcomstaging.com/tag/tn/
https://atomic-temporary-10880024.wpcomstaging.com/tag/tn-visas/
https://atomic-temporary-10880024.wpcomstaging.com/tag/field-offices-in-atlanta-ga-and-memphis-tn/