At the outer marker, if your altitude is higher than the check altitude specified on the chart, you add the difference between the two PLUS the aircrafts pressure error correction .. if not specified, the PEC is assumed to be 50ft...Therefore the lowest minima should be at all times 270ft, unless the PEC shows otherwise..If your altitude shows lower than the specified altitude at the OM, you simply use 220ft ( standard or whatever the chart specifies ) and add the aircraft PEC...IF THE AIRCRAFT IS HIGHER...YOU ADD......IF THE AIRCRAFT IS LOWER...YOU LEAVE THE MINIMA...!! But remember the aircraft PEC..!!!