Pinarello Seat Post & max height extension

I have just bought a Pinarello second hand and it has come with a carbon seat post. It is going to take a massive shim to fit my existing 27.2 post but will need to have the carbon one a long way out to get the right saddle height. Problem is, there is no min line on the post so I do not know how far up I can take it. The post is approx 350 long which I think is standard. Any help on how much of a carbon seat post needs to stay within the frame?
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