I know of no code guidance for this issue. I have developed my own parameters:
New construction -- 5% overstress allowed
Verifying existing construction -- 10% overstress allowed
Justifying my own mistakes after something is already built -- 15% overstress allowed
Remember that loads are in general conservative, that structures are in general redundant, and that our methods of analyzing structures are generally oversimplifications of the actual behavior of structures.
If you are uncomfortable with allowing overstress, look for ways to justify the existing structure (e.g., are there other load paths you are not accounting for?). DaveAtkins