My keg doesn't have a pressure gauge. Should I drill a hole in the top of the keg and fit a brass nipple to the top of the keg, then connect some air hose to a gauge? Or does the air hose that connects to the gauge have to be submersed in the beer itself to get an accurate reading? In other words, do I need to get a reading of the air pressure in the the head space between the top of the beer and the top of the keg or do I need to find out the pressure of the actual liquid inside the keg?