The proper way to charge a newer cars A/C if low is to take it to a shop that has the proper equipment. were they will hook system up to a machene that will recover refrigerant, weight it to find out how low system is, then evacuate system, check for leaks, and then charge system back up with a electronic charging scale putting the exact amount in in ounces. This cost about $100 dollars or less, and everyone is safe.