It's under the heater element, I had it turned around at first so it was reading low but it only went up a few degrees when I turned it around properly. I believe I calibrated it correctly, ice in cold water and it read 32 degrees. I'm not hugely worried if they hatch a little early, Bieles aren't excessively fragile as chicks. And like I said, worst case I try again in a week if none are developing.
The thermometer should be at the same height of the top of your egg's.