For the TPMS system to work properly you HAVE to set the target pressures from time to time. You do this when the car is running but STOPPED and you use the iDrive or BC stalk.
I also think the TPMS light goes off around 20% low. I think it also will trigger when a sudden loss of air pressure is detected. Its a dual system so to speak.
Why not just set your target pressure and then let air out of one tire until the TPMS light come on. Then you know for a FACT when it will trigger.