I have seen different values used for gamma calibration. However, it depends on the isotopes involved. A 'newer' plant uses Co-60 as the main isotope, for thumb rules. Calibration sources are almost always use Cs-137 due to the 30 year half life. If the calibration source (Cs-137) is > 30 years old, you may need to recertify it and run new calibration curves as it can impact the calibration process. For standard gamma calibration use the Cs-137, others have provided the reference in previous posts.
For conversion for beta included (as in a frisker), it is different.
At a former plant, we did a study and found our conversion factor for friskers should have been around 45000 dpm due to the isotopic mix but were using 100000 dpm that commerical nukes seem to favor.
In operating plants isotopic mixes can change (Co-60 reduction, C0-58 generation, and ingrowth of transuranics from forgotten fuel leaks). As the plant ages, hard to detect can become a significant issue.
In decommissioned plants the gamma goes away, while beta and alpha grows. Harder to detect and calibrate instruments with those changes.