I don't know what the right position for you is, but if you're looking to move your seat forward, there are seatposts especially for triathlong bikes that have forward-set clamps. Nitto made an interesting one, Profile Designs makes one, and Redshift Sports makes a "convertible" seatpost that flips between a conventional road position and a forward triathlon position. Not sure if any of these reach 410 mm extension.
Also make sure that whatever seatpost you get has the right diameter.