Murphy's law states that no matter how careful you are in scheduling, something will come up forcing you to go out of town on the scheduled hatch date.
That if you hatch mutt eggs the temp and humidity will stay perfect and you will get a 100% hatch
But put in some valubable hatching eggs and you will have to sleep next to the incu just to keep the temp & humidity right.
Then you will be lucky if you get a chick to hatch on day 21
You check the temperature and humidity faithfully on day 1-18. Figuring you're safe after you take them off the turner, you don't check for a whole day, and when you do, you realize your temperature dropped down to 94 degrees and stayed there.