What is customary diligence?

Answer 1

Customary diligence is a national standard that's applied to certain types of legal writing. For instance, attorneys who issue "no litigation" closing opinions are bound to write those opinions to the standard of "customary diligence" as set forth by the TriBar Opinion Committee in 1998 and the continuing review of the Legal Opinions Committee of the ABA Business Law Section.

Answer 2

The phrase I've usually seen is "due diligence". It basically means spending the necessary time and effort to ensure that something is done properly.