I've bitten the bullet and ordered a 350mm Cinelli Dinamo seatpost. It was hard to find a white 34.9mm seatpost and I got a cracking deal on it - almost 50% off the recommended retail price. I also liked the single-bolt clamp on it, and it has the same 15mm offset as my current stock seatpost.
I've made the assumption that the measurement is from the bottom of the seatpost up to the middle of the saddle clamp.