An egg does not have a day’s worth of development 2 seconds or 2 minutes after you put it in the incubator. It takes 24 hours for an egg to have a day’s worth of development. So when you are counting you say “1” the day after you start them. An easy way to check your counting with chicken eggs, the day of the week you set them is the day of the week they should hatch. Since you set your eggs on Saturday they should hatch on a Saturday. Saturday April 15. Lockdown is Wednesday the 12th.
I agree you don’t have to be that precise. Your goal for lockdown is to get the humidity up before they external pip. It’s not all that unusual for the eggs to be a day or even two days early or late, the 21 day thing is a target that is not always hit. So if you lockdown three days before they hatch you meet this deadline even if they are early.
Good luck with the hatch.
