Normal level of alveolar crest below the CEJ of the adjacent teeth is:

ALVEOLAR CREST: The gingival margin of the alveolar process that extends between the teeth is apparent on radiographs as a radiopaque line—the alveolar crest. The level of this bony crest is considered normal, when it is not more than 1.5 mm from the cementoenamel junction of the adjacent teeth. Reference: White and Pharoah's Oral Radiology Principles and Interpretation 7th edition page no 133