The old standard was to measure either to the center of the top tube or the top of the top tube. Once sloping top tubes and materials other than standard diameter steel tubes came into play, new systems were developed. For example, Serotta always used a nominal size that was based on a level top tube measured center to center, even if the frame had a sloping top tube. So a "60" cm sloping top tube frame might have had a seat tube that measured 54 cm. The idea was that the frame was designed to fit like a traditionally made 60 cm frame. I think the Cannondale system was based around the same idea. A 56 cm Cannondale should fit more or less like a 56 cm traditional lugged steel frame. An argument can be made for sizing by top tube length, especially for frames with severely sloping top tubes.
The Slice Ultra was the 2nd tier fork behind the Premium. It has an aluminum steerer tube while the Premium is all carbon. R700 doesn't give up much to the upper models. CAAD7 frames are nice, weigh only 30g more than the CAAD8.