[CR]Legnano Gran Premio: minimum seatpost insertion

Can anyone tell me what the minimum safe insertion depth would be? How far below the bolt should the seatpost extend? The bike in question has the 3 piece binder bolt in front with no slot, identical to that shown here (not mine):

http://s343.photobucket.com/albums/o476/chileros_bikes/Legnano%20Gran%20Premio%201966/?action=view&current=100_1009.jpg

Thanks,
Ryan Hildebrand
Costa Mesa, California USA