The distal end of the occlusal plane should be positioned at two-thirds up the retromolar pad because this ensures proper support and stability for the mandibular denture. The retromolar pad is a critical landmark composed of mucosa, glandular tissue, and muscle attachments that remain unchanged after tooth loss. Positioning the occlusal plane too low or at the base fails to utilize this anatomical support, while placing it too high may cause discomfort or instability. Thus, the two-thirds height provides an optimal balance for functional occlusion and denture retention.

Reference: Contemporary Prosthodontics, William R. Laney, 5th Edition.