I have three seatposts of different lengths from different makers in front of me here at home.
Aluminum, 300mm, insertion 100mm from bottom
Steel, 360mm, insertion 80mm from bottom
Aluminum, 400mm, insertion 100mm from bottom
From this very small sample, it seems that insertion point might vary from post to post depending on lengths, materials (as evenslower hinted at), or the whims of the manufacturer. You should probably email the manufacturers of any posts you're interested in to ask them for specs.
Last edited by daveizdum; 08-25-09 at 01:17 AM.