Newbie here. I calculated my size on wrenchscience, and it recommends a "center-to-center" 50cm height, or center-to-to 51cm. How does this correspond to size? I am looking at the Trek Pilot 2.1, which has the relaxed geometry, which I guess complicates the measurements a bit. On the trek website, I see an "EFF Top Tube" measurement, and a "Top tube" measurement, that differ by about 1.5 cm. (EFF on the small-52 is 53.1, top tube is 50.8). Does this sound like the right size? I know it is more about what size feels right, but I've never ridden a road bike, so it's hard to tell what feels right. Also, how come the XS size (50) has measurements that are only about 1cm smaller? Is the "frame size" a meaningless number? As a frame of reference, I am about 5'8" with a 30inch inseam - I have relatively short legs and a large head, so I am bit "small" for my height. Thanks a lot.