The objectives for the extension of the proximal margins are all except:
The objectives for the extension of the proximal margins are to:

Include all caries lesion, defects or existing restorative material.
Create approximately 90o cavosurface margins (i.e., butt-joint margins). 
Establish (ideally) not more than 0.5 mm clearance with the adjacent proximal surface facially, lingually, and gingivally.

The location of the final proximal margins (i.e., facial, lingual, gingival) should be established with hand instruments (i.e., chisels, hatchets, trimmers) and not the No.245 bur, so as to prevent unnecessary overextension.
Reference: Sturdevant operative dentistry. 7th EDITION,  Page No:341
Include all caries lesions, defects or existing restorative materials
Create 90-degree cavosurface margins
Establish not more than 0.5 mm clearance with the adjacent proximal surface facially, lingually and gingivally
The location of the final proximal margins should be established with No.245 bur