As you have observed you often don't need to place the AC very far from the structure causing the magnetic anomaly. Even on highly reinforced concrete structures a hand launch will usually get the AC high enough for normal compass operation. The MOD value is your friend in these circumstances.Anytime I am on concrete of any kind I am always thinking possible issue. I always use my Case Club Case to take off on top of and have not had any issues with metal or Rebar. Just enough of an insulator to get you off the metal. Parking garages are notorious for Rebar in the pavement.