It all depends. I have two sons. My older boy has been to state 3 of his 4 years. He placed last year. He could easily be called a AA. Yet, this year he has yet to finish higher than 4th, each tourney having 3 to 5 state placers from last year. That would make him a C. Neither is probably accurate.
Last year my younger son placed at USJOC, won 6U at Ottawa, and was 5th at Districts wrestling up. He might be called AAA. Yet this year, in 4 tourneys, he has only placed once. That would make him a D. Neither is probably accurate.
In my view, for open kids, you try to decide if your wrestler would be in the top 8, 16, 24, 32. Apply A,B,C, and D. to each of these numbers. Regardless of the number of wrestlers in a tourney, it should work out in the end.
Novice is a little tougher. In addition to above, if you pretty well know he will go 0-2 in every open, put him in novice. Regardless of how many years he has wrestled. Don't let a kid lose every match he wrestles.
