Back in the olden days, we had something called a horizontal top tube, and bicycle sizing was fairly simple, you sat on the bike, if your privates cleared the top tube, feet flat on the ground, the frame was the right size, you adjusted the saddle so there was a fist full of seat post, and you were done. For a Mixte or a Ladies frame, all the dimensions were the same with the exception of the top tube. In other word if you took a men's frame and a ladies frame at set one on top of the other, they would be the same size. This all changed with the invention of the sloping top tube, because they tossed the rules of sizing out the window. Some bicycles were measured from the actual new location of the top tube, others were measured from where a horizontal top tube would be, if there was one. The seat tube never was a good place to measure, because seat height is not dependant on it, and different manufacturers use different ratios of top tube length to seat tube length. We need a new bicycle measuring system, that is well documented.