I forgot to mention that you really also need to factor in seat tube angle because you'll likely move you saddle forward to adjust for a seat tube angle that puts you further back. I read somewhere that each degree of seat tube angle difference equates to about 1 cm of effective top tube length. For instance, the 54cm Cross Check has a 73 degree seat tube angle and a 560mm effective top tube, while the 56cm has a 72.5 degree seat tube and a 570mm effective top tube. Comparing apples to apples, the 56 would "effectively" have a 565mm top tube compared to the 54, because you'd slide your seat forward 5 mm more. Of course, this only helps if you know the seat tube angle on your Pinarello.