The seatpost seams to have an angle as well. I would check as well the seat tube angle of the Pinarello. It may have a wider angle, in comparison then the X-33 seat will be a bit more backwards. Maybe you can find the info on Geometry Geeks and do a comparison there. Personally I would go 540 and you could still get a slightly longer Stem in case, but it is guessing without seeing the Pinarello geometry....