Thinking about buying this or a Thomson seat post for my caad9 build:

http://www.artscyclery.com/descpage-PROV7SP.html

Not sure what length to get. I've determined I'll need approximately 7 inches or about 178mm of seat post extension between the top of the seat post clamp and the seat rails.

The problem is I don't know what the typical minimum insertion length is for a typical aluminum seat post. If I get the 280mm length that leaves approximately 10cm of insertion into the frame. Is that enough for a 180 lbs guy like myself? Also, when they say 280mm I don't know if that's usable seat post or the entire length from the base to the top of the clamp.

Can anybody help me out here so I know which post to buy?