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
0.8 mm ; two thirds in dentin and one third at the expense of enamel
0.8 mm ; one third in dentin and two thirds at the expense of enamel
0.8 mm ; full depth in dentin
0.8 mm ; full depth in enamel