Well, I just hatched my first ever batch of eggs recently and before I started to incubate them I was puzzling about that exact same thing. I counted from right when I put the eggs in, so the first 24 hours in the incubator was day 1. I started them on a Friday evening, so the end of days 7 & 14 was the next two Friday evenings, and the 24 hours of day 18 was from Monday evening through to Tuesday evening. I wasn't sure about when to go into lockdown either, but I took a guess and stopped turning them on Tuesday morning, halfway through day 18. Mine ended up running late though. Nothing happened on day 21 and only one (out of six) pipped on day 22. And I was getting all despondent and thinking I must have done something terribly wrong. But by the end of day 23 I had four chicks. One died after pipping and one didn't pip at all. I was happy with 4/6 for a first try though.
From what I've read, the regular turning of the eggs is absolutely vital for the first 7 - 10 days, but not so important after that. Like, if you don't turn your eggs in the first week, you'll get no chicks at all, ever. But if you turn them for the first ten days and then stop, you might still get a few hatching out okay. That's just from what I've read on other forums, from people whose auto-turn in their incubators broke halfway through the incubation, and they couldn't take time off work to turn their eggs by hand. And they got fairly decent hatch rates anyway! So maybe a few hours here or there on day 18 isn't that big a deal after all...
That's just my thoughts btw, I really don't know for sure. Hope you get some replies from more knowledgeable folks!
And good luck with your hatch!
