I don't usually go to conferences, but this summer I attended one. It was not open source specific conference, although there were some open source projects there. As far as I know, there were around 150 attendees, I counted exactly two females. To me that 15% sounds quite a bit inflated, but maybe nowadays in the US that number could be close enough to the reality.
Based on my own corporate experience, it seems about right to me. My wife worked for a non-profit trying to encourage high school girls to start careers in STEM and that was an often used figure it seemed.