The ideal cavosurface angle for an amalgam restoration is 90° because it provides a right-angled margin that offers optimal resistance to marginal breakdown. A 90° angle ensures the restoration has enough bulk and strength at the margins to withstand occlusal forces without chipping or fracturing. It also facilitates proper condensation and adaptation of the amalgam material, leading to a durable and long-lasting restoration. Angles less than 90° can create thin, weak margins prone to failure, while larger angles reduce retention and increase enamel removal unnecessarily. Therefore, maintaining a 90° cavosurface angle is crucial for the longevity of amalgam restorations.

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