Where to place my thermostat probe in my incubator?

I am making a mini fridge into a incubator and I am just curious where I should place the probe from the thermostat inside the incubatorthe measurements are 30 tall 18 deep and 18 wide. Also when I add the thermometer prob should that just be placed right around the same spot as the thermostat probe.Thanks

just put it about 3 inches below the surface of the vermiculite. sinse this should be the deepest the eggs should go,