tbh to achieve a perfect fit the original seatpost is only 7-8mm above the minimum line so while not ideal for riding.
I advise you to stop riding it until you get a longer seatpost, Cannondale will not honour the warranty if the frame cracked near the seat clamp (it happened more than once on both carbon and alu model).
Good advice, I’ve not actually ridden in ‘bigbird’ setting, only sized it against my commuter which does fit. Seems cannondale offer 400mm C2 posts but also found a hylix carbon post from Taiwan on eBay, I’ll see what turns up over the coming weeks before possibly taking the latter option.
I advise you to stop riding it until you get a longer seatpost, Cannondale will not honour the warranty if the frame cracked near the seat clamp (it happened more than once on both carbon and alu model).