Does this bike have a horizontal (traditional) top tube? If it is a modern, sloping top tube, you have to measure from where a traditional top tube would hit a longer seatpost. (I don't know if center-to-center and center-to-top terminology is still used. If it were me, I'd go c-c. I'd measure center of the top tube/headset intersection to the ground, then mark that height on the seatpost. Now measure that new bark to the BB.)

Ben