Here is one I am trying to figure out. I have a homemade cabinet style incubator and hatcher. I have been having a issue with the chicks hatching out a day early, thru the last five hatches. TEmps are spot on with a range of 99.2 to 99.7. Incubator side I run at 40% humidity. The eggs are set in the hatcher side at day 18 with humidity at 60%. My hatch rates are never below 90% and all chicks are healthy with no defects. The incubator and hatcher both run with air circulation via a computer fan which is set on medium speed. I have calibrated all temperature gauges and mercury and they are spot on. After reading thru several threads early hatches are from high temps. I was thinking slow down the fan speed?