Top tube, I go from center of the head tube where the top tube or tubes for a mixte meet, or about an 1-2" down from the top of the head tube, to the center of the seat tube where the rear stays meet. Seat tube, seat tube where the rear stays meet to the center of the bottom bracket. Or as close to those points as possible.

I mistakenly bought my wife a mountain bike that was a little bit on the smallish side, and it makes here sort of cramped up on it but she liked the ability to put here feet down. Like a 15" men's frame. I recently built up a 90's-ish hybrid from scratch for here that was more her size. it was a 19" size frame. Go figure, she is 5'5" by the way.

Women's frames are weird, and I actually think that the more modern stuff, 90's on, seem to fit more realistically. I also just think that women are more difficult to fit up properly. Sometimes I will size ladies in men's sizes as a base point first and then go backwards to find a low step frame, if it is what they want, that will fit properly.