While preparing the proximal box, the mesiodistal width of the ditch should be:

Proximal Box If the bur is allowed to cut only dentin, the resulting axial wall would be too deep. The mesiodistal width of the ditch should be 0.8 mm (the tip diameter of the bur) and prepared approximately two thirds (0.5 mm) at the expense of dentin and one third (0.3 mm) at the expense of enamel. Sturdevant operative dentistry 7th Edition, Page No:e99