How is seat post length measured?

richtw Posts: 118
edited November 2010 in Road buying advice
Does anyone know how the seat post length is measured on Ritchey posts?

I have a post in the bike which I am looking to change 31.6mm diameter, total length of the post is about 360mm from top of clamp to bottom of post.. The bit of the post that states the length is scuffed and unreadable.

Ritchey sell posts in this diameter of 300 and 400mm - the post I have is neither length. So how do they measure them?