Everyone will have different outcomes, you will have to adjust many factors for your particular area, but egg quality makes THE difference. This summer my daughter and I worked from a 53% hatch rate to an average 93% hatch rate with varying egg quality. We didn't count yolkers, but we did count quitters in our percentages. My daughter had 3 100% batches, all with eggs from mixed farms known locally for quality, and 2 with turken eggs mixed in (all turkens hatched 1 day before the chickens in all instances) all sets had no more than 16 and no less than 10 eggs. We incubated them in an original Hovabator from the '80s, turning only 3 times per day, by hand. (We did do one small set of 6 that we only turned 2 times a day, by mistake, and hatched 100%)
The winning combination from the best sets; Humidity was maintained at 45-55 and it did fluctuate, we candle at 10 &18 days, humidity 45-55 in lockdown and 70-75% after 1st crack. we didn't need to add water after lockdown most sets as the hatching eggs bumped it up a ton. The absolute coolest and best thing we did, and my wife was the 1st to do it, was to take eggs left 1 day after the majority hatch and just crack 'em open. Many times we had 2 or 3 that just would not try. If you candle before lockdown then all viable eggs going into lockdown should hatch. "Helping" a few out made the difference and in all cases these chicks survived, and one became our pet, Gertrude. What we did was take a pencil and lightly tap around the egg where they would normally cut...wait an hour or so...and if the chick didn't try to push out, we started peeling back little by little...the egg will bleed, stop pulling back when bleeding starts...and unlike me, don't freak out. If you go slow...like 4-5 hours in Gertrude's case, you can little by little hatch them.
Sounds oversimplified, but we did it all summer and found that the less you do, the better. Temps and humidity fluctuate naturally, and they do in the incubator, so don't waste time trying to maintain constants. For our best success we obtained quality eggs no more than 7 days old, set the eggs, maintain a range of temps and humidity that worked for our area, didn't "over" turn them, and helped them if they needed it.
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