So you get a very accurate thermometer and put it in the incubator to see if it's reading the same temp as your incubator. If it's not reading the same temp.then your incubator is out of wack.so you just adjust your temp up or down as long as the thermometer is reading 37.6 inside the bator then that's okay.
Yes, Putting more than one very accurate thermometer would be recommended since sometimes they read wrong that way you have more to read off of incase one is bad.