It starts with the question "which size is acceptable". There is no standard; some companies used the same values, other companies other values. And your personal acceptance limit is another thing and most probably depends on the size of the print as well.
Originally Posted by Peltigera
DoF calculators often use the most used value or even allow you to set your own CoC.
If you look at say 50mm lenses from different manufacturers you will find a variety of DoF scales.
DoF is mainly subjective. "playing safe" implies an objectivity which is simply not there.