that ice cube method seems ridiculously like back yard science.
I know this is called backyardchickens but that just reminds me of my sister saying to lick sweets clean if you find them on the ground as you shouldn't eat dirty sweets.
Ice cubes are -18 degrees. Tap water is 4 degrees, mix it, let it sit for 3 minutes, fart in it a few times, it is just pure guess work what temperature it will be and will probably mis-calibrate any thermometer more.
I would trust your thermometer more than the incubator temperature. Certain models are known to run cool.
Turn the incubator up to 105. Buy another thermometer if you are uncertain. I would not trust a strangers advice for my eggs personally so although I am pretty sure it is the incubator that will be most off you should maybe get a aquarium glass thermometer to calibrate with, they are about $3.
Jesus now there is a backyard cooler calibration method online too? Just to point out how flawed these methods are, lets say you do it on a warm day, its 30 degrees out, your ice is -18. When do you know when the temperature is exactly between the two? We know the ice will eventually after about 12 hours turn to water. So you start at -18 degrees and after 12 hours you end up at roughly +15. When is the exact time to measure it so it is exactly 0.0 degrees? After 3 hours? After 3 hours and 13 minutes?
What does the aricle say about that? What bout if you do it in winter on a really cold day when it's only 15 degrees out. What does the article say to do then?
I can tell you what it will say, it won't mention any of this because it is flawed.
There is one scientific method I know of where one can measure a certain temperature and be sure it will be that temperature. And that is at the bottom of a frozen pond. So what you need to do is place your thermometer in an airtight bag, have a pond that is ideally at least 2 meters deep. Wait for it to freeze over completely. It has to be completely frozen. Then drill a hole in the ice. Lower the thermometer to the bottom of the pond. Leave it there for 2 hours. When you pull it back up again the temperature should be exactly 4 degrees C as Water is heaviest at 4 degrees C so as a pond freezes and it reaches 4 degrees that water sinks to the bottom. This is why fish can survive freezing ponds as the 4 degree water will sink to the bottom as it freezes and keep it unfrozen.