The next time you get a signal you think is deep iron lift the coil while still swinging. See if the meter numbers go from negative to positive. Also try it over deep nonferrous targets and see how much the meter numbers change. By chance, I tried that yesterday and I saw deep iron numbers change from negative to positive as the coil was raised, while deep nonferrous numbers didn't change much. I have to try that again next time out. It might help prevent digging deep nails.