I don't think it matters as long as the air cells are decent. Eggs technically don't have to be turned past day 14 and if they are developmentally delayed more than the 4 days, chances of them making it are so slim. The only thing pushing back lockdown is going to really affect at this time is in the area of humidity. If the air cells are normal for 18 days gestation, then upping the humidity isn't going to change that. If they aren't sufficient for 18 days then. yes, pushing back lockdown would be beneficial. That's the only reason I would I would hold back at this time though.