That makes sense.

I looked up two bikes - Pinarello Paris and the CAAD10. The Paris, 51.5, has a 53.5 ETT, 13 cm head tube. The CAAD10, 54, has a 54.4 ETT and a 14 cm head tube. You need to take into account the head tube so your stem doesn't end up too high or too low.

I'm not sure what your goal is with fit - is the CAAD9 a bit off or are you trying to replicate that fit?

With head tube heights I consider any integrated head tube length about equal in terms of functional length to other integrated head tube lengths. This is based on the fact that most of them require a few mm minimum of spacers/caps on top and most of them hold the fork in about the same place. In other words a 14 cm integrated head tube is about the same size as a 14 cm integrated head tube across a wide range of manufacturers.

For me when I was looking to replicate a position (actually I wanted the longest possible top tube) I rode, depending on the year, a size S Giant TCR or a 52 cm SystemSix (aka similar in geometry to a CAAD9). Both had 53.5 cm ETT, both had similar head tube lengths (the Cannondale was shorter I think), but the TCR had a much shorter seat tube height (40 cm to the top of the top tube on the aluminum frame, 44 cm to the top of the top tube on the carbon one).