The ideal clearance of 0.5 mm between the gingival floor of a preparation and the adjacent tooth surface is necessary to maintain adequate proximal contact while allowing proper access for restorative material placement. This clearance ensures prevention of food impaction and helps maintain periodontal health by avoiding plaque accumulation in tight, inaccessible areas. Too little clearance (0.2 mm) risks inadequate material thickness and contact, while excessive clearance (>0.5 mm) compromises tooth structure and contact integrity. Thus, 0.5 mm is the standard for functional restoration without harming adjacent teeth or gingiva.

Reference: Sturdevant's Art and Science of Operative Dentistry, 7th Edition.