P is already used, of course. But read my AFAIK more carefully.
It is not "my" proposal. It is used in the SI booklet (English translation) edited by Chester H Page of NBS and Paul Vigoureux of NPL. Paul was a friend of mine.
For an example, see Table 3 of .
See also Section 1.2.3 of Cohen and Giacomo 1987, possibly accessible via