That is the way I figure my hatches as well. From the day of the week that they go in, to the same day of the week 3 weeks later. Mine usually hatch/start hatching on day 21. I have had them hatch as much s 3 days late, but they don't usually do well in those instances.Hmm...I wonder if I'm counting my days wrong. I've had 3 hatches now where they start pipping the day before I expect them too. I set eggs after work on a Friday and expect to hatch on Friday 3 weeks. Is that wrong? Also I've left eggs up to 3 days in the hatcher with no new birds.