The pin channel should be prepared 0.5 mm inside the dentinoenamel junction (DEJ) to ensure adequate retention while preserving tooth structure. Positioning the pin channel slightly within dentin provides optimal mechanical strength and avoids enamel brittleness, reducing the risk of enamel fracture during pin placement. This depth also ensures the pin engages sound dentin to support the restorative material effectively. Preparing too close to or outside the DEJ compromises retention or weakens the enamel. Therefore, a 0.5 mm depth inside the DEJ balances retention, strength, and tooth conservation for successful restoration.

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