It can be confusing. It seems that the day you put them in the incubator should be Day 1, but that is not right. An egg does not have a day's worth of development 2 seconds or 2 hours after it is put in the incubator. It takes 24 hours for the egg to have a day's worth of development. Lockdown should be after 18 days of development. Hatch should be after 21 days of development.
An easy way to determine hatch date is that they should hatch on the day of the week you set them. You set them on a Thursday. They should hatch on a Thursday. Lockdown for you should be Monday the 21st.
What makes this even more complicated is that this is theory. Actual practice can be different. When an egg hatches, it is not an instantaneous process. It can take hours, even more than a day. And many different things can affect when an egg actually hatches. Heredity, humidity, size of the egg, how and how long they were stored before starting all play a part, but the big one is average incubating temperature. If you incubator is running a bit warm, they can hatch early. I've had them pipping when I went into lockdown due to this. They still hatched. If your incubator is a bit cool, they can be a few days late. This is normal. In a still air incubator, you might get different temperatures in different parts of the incubator.
Lockdown involves stopping turning them and raising the humidity. You do not have to be real precise with lockdown. If you are off many hours it probably won't matter. The key is to try to raise the humidity and go into lockdown before they start to pip to reduce chances of shrink-wrapping them. Eggs don't always get shrink-wrapped if they pip before you raise the humidity, but it can happen. I've shrink-wrapped some. I've also had several hatch that had pipped before I raised the humidity. Raising the humidity before they pip improves your odds. It does not guarantee success or failure.
Since this is your first time with that incubator, I suggest you go with the "traditional" lockdown of after 18 days of development. Once you get some history with that incubator you might want to adjust this.
