I think the recipient is the whole *point* of ratings - ratings are an attempt to protect the recipient from content that is not appropriate for them.
If I say this fic is 'worksafe' I'm talking about the recipient - if you, the recipient, are at work, you probably won't mind being found looking at this by your boss. Except, of course, that the rules at my work may be different than at your work.
If the picture shows a naked penis, I could add a warning 'full-frontal nudity' because warnings are about content, but if I say it's rated 'Explicit' then I'm making a determination as to who the recipients should be.
Michelangelo's David, unless it's been fig-leafed, has full-frontal nudity - there's no question about that. But whether it's considered Worksafe, Standard, Mature or Explicit depends entirely upon the current social mores. The fig-leaf is the rating, in a way, and it's been put on and taken off at different times over the years.
Ratings are always about social customs - "most people would find this offensive" , "most people wouldn't mind their children seeing this" - there's always a "most people" which means that "some people" don't agree.
Warnings are about the content, and can be either objective or subjective but Ratings are about the recipient, and always depend heavily on community standards, which are by no means universal either across time or between societies.
Ratings are the overall "level;"
But, level of what? I've seen level of 'intensity' or level of 'maturity', but it all comes down to what "most people" are comfortable with.
I actually like 'Worksafe' it implies that this is something you'd be okay caught reading in the most conservative of places. That is, that the content is 'comfortable for everyone'. Worksafe is probably similar to 'General' - appropriate for all audiences.
Your 'Standard' seems to be an 'appropriate for most people' - there may be a few exceptions - young children, people who are easily offended, etc. but 'most people' would find it okay.
'Mature' is more like 'many people' - not everyone is comfortable with this, in fact, there are quite a few people who aren't.
And 'Explicit' is 'some people' - most people aren't comfortable with this, or at least, not in public.
In other words, no matter what you call them, the four ratings will always come down to:
Suitable for 'Everyone', 'Most People', 'Many People', & 'Some People' . Maybe you should just call them that :)