In an ideal Class III amalgam restoration, the filling should extend 0.2-0.3 mm into the facial embrasure to ensure proper contact and contour with the adjacent tooth. This minimal extension preserves the embrasure form, maintaining gingival health and interdental papilla integrity, while providing sufficient bulk for amalgam strength without overcontouring. Excessive extension risks overhanging margins, which can lead to plaque accumulation and periodontal issues. Therefore, a precise and conservative extension balances restoration durability with periodontal health.

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