I am NOT asking for the percentage of chess players that are women.
I am asking for the percentage of players who are women in the sections where both men and women can play in.
That percentage is obviously lower than the percentage of chess players that are women because many women prefer to play in women-only sections.
In other words the question could be rephrased as: if I'm a man and tomorrow I'll play a game in a tournament, what's the probability that my opponent is a woman?
You can answer for either USCF or FIDE, it doesn't matter which one.