Landmark for pup chamber opening is :
CEJ
Cervical third of crown
Level of alveolar bone
Enamel
Krasner and Rankow determined that the CEJ is the most important anatomic landmark for determining the location of pulp chambers and root canal orifices. They demonstrated  that  specific  and  consistent  pulp  chamber floor and wall anatomy exist and proposed laws for assisting  clinicians  to  identify  canal  morphology.
Ref: Grossman endodotic practice 13th ed page no 248