Re: Lowest common or 90th percentile
To be honest, I haven't the faintest idea what you mean! I don't intend to waste server space by replying with no real message, but perhaps if you asked your question in a more simple way people might be able to answer...
A first grading (at least in our organisation) is usually just an acknowledgement that you understand basic dojo etiquette and can perform easy ukemi- this grade is "kihon" (again, at least in our organisation), and the next is 6th Kyu, (my current grade) where you have a basic understanding of techniques like kotegaishi, kokyu-nage, suwari waza kokyo ho and ikkyo, although you train more advanced techniques when they're being taught.
There are 2 6th kyu students in our class at the moment- we both graded on the same night at the start of the year, and the other guy is better than me at most techniques (he's a yudansha in jiu-jitsu, though I don't think he still trains in it). There are 5th kyus who were better than me when they were 6th kyu, etc. Grade, IMO, is only truly reflective of skill in the higher levels. As the wise men say, it's all about the person, not the grade. I don't believe it's possible to judge skill purely by grade- although after 1st kyu, grades are awarded on basis of skill...
I really don't know if that made any more sense than the question, or whether it was helpful. Oh well.