I would like to share my thoughts on temp and humidity. First, incubation techniques will vary depending on the climate where you live and the type of incubator you are using. Altitude, average humidity, etc. will have an effect on your particular setup. This is based on my local climate here in Texas using a GQF cabinet incubator.
Humidity control - I try to average 45% humidity for the first 18 days. If it drops below 40% or rises above 50% during that time I will make an adjustment. Once they are moved to the hatcher the humidity is raised to 60%. As the chicks begin to hatch, the humidity will rise to 65-70% on its own due to the wet hatching chicks. I have had great success with this method. Very few if any chicks that get stuck in the shell.
I use a digital hygrometer but only after it has been calibrated. To calibrate, pour about 1/4 cup of table salt in a cup and add just enough water to make it like wet sand. Put it in a gallon zip-lock bag along with the hygrometer and seal it up for a couple of hours. The humidity should read 75%. If it doesn't then just add or subtract the difference and write it on the unit. (Ex. If it reads 67% then write +8 on the unit)
Temp control - The most accurate thermometer I have found is a digital thermometer for humans. It can be purchased in the pharmacy at wal-mart. I drilled a small hole in the side of the cabinet about half way up and insert and just leave it there all the time. I only turn it on long enough to check/adjust temp and then turn it off. I try to keep the temp between 99.4 and 99.9 degrees. This thermometer is so sensitive you can adjust by .1 degrees.
Cabinet incubators are not a closed environment. They have air vents that constantly circulate and exchange the air inside the unit. The incubator temp and humidity will change with the room temp and humidity. For every 10 degrees of room temp change the incubator temp will change by 1 degree. The more stable you can keep your room the more stable the incubator will be.
I hope this helps. Feel free to share your opinions and experiences…