Originally Posted by rufus
yes. but what my initial puzzlement was, if the seat tube angle, and top tube length are the only variables, a steeper seat tube angle shortens the reach(top tube length). to compensate for this shortening, you then use a seat post with more setback, and move the saddle further back on the post. but now, we're not just talking how seat angle changes affect reach, we're introducing other variables as well.
That's why I eliminated the variable of the top tube in my example. You do not move the saddle backwards to make up for loss of reach, you use it to adjust for the rider's relationship to the BB. I can't address the other issues as I only have a basic understanding of frame geometry--just enough to get by in choosing a well fitting frame.