The isthmus width for an inlay preparation is generally 1/3 of the intercuspal distance to ensure adequate strength and resistance form without excessive removal of tooth structure. This dimension maintains the structural integrity of the tooth while allowing proper clearance for restorative material. A width less than 1/3 may compromise durability, while wider preparations (>1/3) increase the risk of weakening the cusps. Thus, 1/3 intercuspal distance balances conservation and mechanical stability of the inlay restoration.

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