I think this is apparent if you study the comments above in detail, but I noticed in the documentation for the PrincipledBSDF in Blender it states clearly:

When converting from the older Glossy BSDF node, use the square root of the original value.

That seems to apply here too. So, to convert a P11 or earlier setting to P12, just take the square root of the original roughness. To convert a P12 setting for P11 or earlier, square the value (i.e. X^2, or X*X).