Never trust the guages on an incubator. Stop adding every thermometer you own into it or you'll have a headache.
First and foremost you should have a heat sink in there to mimic mass of eggs. A brick or few good sized stones will work or even a bunch of water bottles. Those will stabilize the heat swings just like a few dozen eggs in the incubator will. If you purchased only a dozen eggs then I suggest you keep a few bottles of water in there to aid in steady heat. Also note it will take hours for incubator to settle on temperature until the mass you put in gets up to temp.
Calibrating. The cheapest and most readily available accurate thermometer is in your medicine cabinet. Use a glass or digital oral/rectal thermometer to fine tune your incubator temp. I put a digital down a vent hole and read the high temp then turn it on and off until I get a low temp and average the two. Low temp is 30 seconds after the heating unit turns on. Temp still goes down until heat element warms up.
There is a salt test you can do to calibrate hygrometers too. So between an oral thermometer and salt test your good to go. I use a small cigar box temp/hygrometer unit in the incubator. The temp only reads to whole degree but it's an indicator if anything is going wrong. I fine tune and make all adjustments during incubating with the oral thermometer. Walmart sells Accurite combo units for little money. Those are good as you get the temp (calibrate) as indicator things are well or not and a hygrometer unit you can fit in a ziplock bag to salt test for calibration.
Fill milk/juice/soda cap with salt and add drops of water until saturated. I pour off any standing water.
Put hygrometer and cap into a zip seal bag and wait at least 4 hours. I provide small pillow of air.
A salt environment at room temp will be 75% RH. Take your reading and subtract 75. That is your calibration.
Ex. your reading is 82%RH, 82-75= -7, you'd always subtract 7 from your readings for true RH.
Write the calibration on masking tape and stick it to incubator as reminder.