I think that Colnago approaches the problem from the assumption that you're going to buy the correct bike for your height.
I own two 57 Colnagos as well as one 58. The reach on them is about .7cm different, and that's driven entirely by the increased TTL. Reason - the saddle is in precisely the same place horizontally and in relation to the BB. The setback between those two frames is different only because the calculation is done with the longer seat tube on the 58. Because the seat tube angle is the same, the saddle is positioned the same relative to the BB and it simply boils down to more seatpost showing on one bike, vs. the other.
The reason I am suggesting that you ignore published setback values is because in the case of Colnago, the seat tube angles change between some sizes and because they are calculated with different seat tube lengths which do not contribute any effect on reach (my example above.) The picture gets confused by that fact and the example you are giving just happens to be exactly where they change.
I think it's easier to just consider the seat tube angle impact on the position of the saddle.
Let's say you're riding a 56 and you're used to the leg position. Now a good 57 frame comes along. To make that 57 fit the same way as your 56 you will have to move the saddle forward by .5CM. The result would be .5cm less reach, effectively reducing the TTL by that amount. If you were happy with the reach on your 56, that's good, because that .5CM forward reduces the longer 57 TTL from 55.6 to 55.1. Close enough.
Going the other way, let's say you're riding a 57 and a nice 56 comes along. To maintain your leg positon on the 56, you have to move the saddle back by .5CM which is good, because you increase the TTL from 55.0 to 55.6. Again, close enough.
Same stem on both bikes, some adjustments needed to maintain bar height due to different head tube lengths.
Last edited by terry b; 11-15-07 at 04:51 PM.