The 'normal' reading for the Entity Sensor is around 350 - 550 mG (35 - 55 uT), depending on where you live.
This reading is the magnitude of the 3 magnetic sensor values (X, Y and Z) in the phone.
According to this
, the strength of the magnetic field at the equator is (310 mg) 31uT, and at
50 deg. latitude, it's 580 mG (58uT).
For detecting ghosts, what you are looking for are spikes where there shouldn't be any. A lot of things around the house will cause high EMF readings -- electrical wires, pipes, motors, toys, magnets, etc.
If you get high readings, check to see if you are near any of these. If not, try communicating, and see if you get responses by seeing spikes in the readings.