I'm thinking of replacing the alloy seatpost on my CX bike with a carbon seatpost in the hopes to add some dampening.

The bike is a Cannondale SuperX and has a Cannondale C3 alloy 27.2mm x 300m seatpost. I already have the saddle pushed back most of the way, so I need to ensure I get a seatpost with at least the same setback.

So how do you measure it?