Mine were not 100% predictable in Jan, but I have had 100% fertility at 10 days since the first of Feb.
For several hundred eggs that's pretty good.
But... The two setting incubators I am using are obviously very different. One has an almost 90% hatch rate and the other only about 55%. And it doesn't matter which pens eggs I put in which one.
So... I have shut down the bad bator - too frustrating. After more careful monitoring it is having a pretty good variable in temp.
I'm hatched out for the season anyway and probably kept way too many. Although culling heavy (some times too heavy), is a trait of mine so eventually most will go.
FYI... Those 24 hour thermometers which give you the high and low over the last 24 hrs come in real handy for monitoring how stable your thermostat is.
Now if Santa would just bring me a cabinet bator... ;-)
I'd love to have only one bator to worry about. Hatching every 5 days is too much to track. I'd rather just do weekly, but needed the 5 day stagger to make use of all the fertile eggs in the space I had.