My humble opinion is that you may be getting things a bit backwards here.
You should be determining the tolerance ratio from your lab before you send your equipment there, not when it gets back. As part of the "contract review" process you must do the due diligence to determine if the lab's capabilities are equal to your requirements, otherwise find another lab. This is covered in section 4.4 of ISO 17025:2005.
As far as the acceptance criteria for the calibration results, that is more a check to see if the results provided meet what you requested from the lab. If there are errors, omissions, tests requested but not performed, results that have a different accuracy ratio from the ones contracted for, etc. then you go back to the lab to get these corrected and resolved. Basically you are documenting that you got what you expected, and it meets your requirements.