Cues to lockdown rather than time

[Snipes]

I have mallard eggs that I have been reading take less time than the 28 days that are usually said to take for duck eggs. They take about 26.5-27 days? I also think I may have been incubating a little hotter than I wanted to, so they might be developing faster. I am worried about locking them down too late. Should I go for cues from candling to determine when to lock down rather than waiting til day 25 (or 24, or 23)? If so, what should I look for?

 

[SarahIrl]

Very little gap between chick and air sac, about 1-2mm of fluid left. My chick eggs looked like that at the end of day 17 and I locked down the following morning. Halfway through day 18.